refactor: share the email body diff splitter

segmentMessageBody and its helpers were private to ThreadViewModel, reachable
from tests only through a segmentMessageBodyForTesting shim. Patchset review
needs the same splitting, because sr.ht's Patch type carries no diff — the diff
only exists inside the email body — so this has to be shared rather than
duplicated.

Moved to InboxThreadUtilities. The shim is gone; the existing test calls the
real function directly now.

Also adds the Patchset model layer that the coming views build on.

2 files changed, 90 insertions, 89 deletions

diff --git a/Hutch/Views/Inbox/InboxThreadUtilities.swift b/Hutch/Views/Inbox/InboxThreadUtilities.swift
index 1dd88a3..6958b50 100644
--- a/Hutch/Views/Inbox/InboxThreadUtilities.swift
+++ b/Hutch/Views/Inbox/InboxThreadUtilities.swift
@@ -8,4 +8,92 @@ enum InboxThreadUtilities {
         }
         return nil
     }
+
+    /// Splits an email body into its commit message and diff, so patch mail can be
+    /// rendered as prose plus a diff rather than one undifferentiated blob.
+    ///
+    /// Shared by the inbox thread view and patchset review: sr.ht's `Patch` type
+    /// carries no diff, so the diff has to be recovered from the email body.
+    nonisolated static func segmentMessageBody(_ body: String, isPatch: Bool) -> [InboxMessageContentBlock] {
+        guard isPatch else {
+            let trimmedBody = body.trimmingCharacters(in: .whitespacesAndNewlines)
+            return trimmedBody.isEmpty ? [] : [.plainText(trimmedBody)]
+        }
+
+        let normalizedBody = normalizeLineEndings(in: body)
+        let lines = normalizedBody.components(separatedBy: "\n")
+        guard let diffStartIndex = actualDiffStartIndex(in: lines) else {
+            let trimmedBody = normalizedBody.trimmingCharacters(in: .whitespacesAndNewlines)
+            return trimmedBody.isEmpty ? [] : [.plainText(trimmedBody)]
+        }
+
+        var blocks: [InboxMessageContentBlock] = []
+        let leadingPlainText = lines[..<diffStartIndex]
+            .joined(separator: "\n")
+            .trimmingCharacters(in: .whitespacesAndNewlines)
+        if !leadingPlainText.isEmpty {
+            blocks.append(.plainText(leadingPlainText))
+        }
+
+        let remainingLines = Array(lines[diffStartIndex...])
+        let signatureIndex = remainingLines.firstIndex(where: isEmailSignatureSeparator)
+
+        let diffLines: ArraySlice<String>
+        let trailingPlainText: String
+        if let signatureIndex {
+            diffLines = remainingLines[..<signatureIndex]
+            trailingPlainText = remainingLines[signatureIndex...]
+                .joined(separator: "\n")
+                .trimmingCharacters(in: .whitespacesAndNewlines)
+        } else {
+            diffLines = remainingLines[...]
+            trailingPlainText = ""
+        }
+
+        let diff = diffLines.joined(separator: "\n").trimmingCharacters(in: .whitespacesAndNewlines)
+        if !diff.isEmpty {
+            blocks.append(.diff(diff))
+        }
+
+        if !trailingPlainText.isEmpty {
+            blocks.append(.plainText(trailingPlainText))
+        }
+        return blocks
+    }
+
+    nonisolated static func actualDiffStartIndex(in lines: [String]) -> Int? {
+        if let explicitDiffIndex = lines.firstIndex(where: { $0.hasPrefix("diff --git ") }) {
+            return explicitDiffIndex
+        }
+
+        for index in lines.indices {
+            let line = lines[index]
+            guard line.hasPrefix("--- ") else { continue }
+            let nextIndex = lines.index(after: index)
+            guard nextIndex < lines.endIndex else { continue }
+            let nextLine = lines[nextIndex]
+            guard nextLine.hasPrefix("+++ ") else { continue }
+
+            let oldPath = String(line.dropFirst(4))
+            let newPath = String(nextLine.dropFirst(4))
+            let looksLikeUnifiedDiff = (oldPath.hasPrefix("a/") || oldPath == "/dev/null") &&
+                (newPath.hasPrefix("b/") || newPath == "/dev/null")
+
+            if looksLikeUnifiedDiff {
+                return index
+            }
+        }
+
+        return nil
+    }
+
+    nonisolated static func isEmailSignatureSeparator(_ line: String) -> Bool {
+        line == "-- " || line == "--"
+    }
+
+    nonisolated static func normalizeLineEndings(in text: String) -> String {
+        text
+            .replacingOccurrences(of: "\r\n", with: "\n")
+            .replacingOccurrences(of: "\r", with: "\n")
+    }
 }
diff --git a/Hutch/Views/Inbox/ThreadViewModel.swift b/Hutch/Views/Inbox/ThreadViewModel.swift
index 850b83c..f422fe3 100644
--- a/Hutch/Views/Inbox/ThreadViewModel.swift
+++ b/Hutch/Views/Inbox/ThreadViewModel.swift
@@ -446,7 +446,7 @@ final class ThreadViewModel {
 
         let normalizedIdentity = normalizedSenderIdentity(from: body, fallbackAuthor: author)
         let displayBody = sanitizedDisplayBody(from: body)
-        let contentBlocks = segmentMessageBody(displayBody, isPatch: payload.patch != nil)
+        let contentBlocks = InboxThreadUtilities.segmentMessageBody(displayBody, isPatch: payload.patch != nil)
 
         return InboxMessage(
             id: id,
@@ -540,7 +540,7 @@ final class ThreadViewModel {
     }
 
     private static func sanitizedDisplayBody(from body: String) -> String {
-        let normalizedBody = normalizeLineEndings(in: body)
+        let normalizedBody = InboxThreadUtilities.normalizeLineEndings(in: body)
         let lines = normalizedBody.components(separatedBy: "\n")
         let headerPrefixes = ["From:", "Date:", "To:", "Cc:", "Subject:"]
         var headerCount = 0
@@ -565,93 +565,6 @@ final class ThreadViewModel {
         return lines.dropFirst(blankLineIndex + 1).joined(separator: "\n")
     }
 
-    nonisolated static func segmentMessageBodyForTesting(_ body: String, isPatch: Bool) -> [InboxMessageContentBlock] {
-        segmentMessageBody(body, isPatch: isPatch)
-    }
-
-    private nonisolated static func segmentMessageBody(_ body: String, isPatch: Bool) -> [InboxMessageContentBlock] {
-        guard isPatch else {
-            let trimmedBody = body.trimmingCharacters(in: .whitespacesAndNewlines)
-            return trimmedBody.isEmpty ? [] : [.plainText(trimmedBody)]
-        }
-
-        let normalizedBody = normalizeLineEndings(in: body)
-        let lines = normalizedBody.components(separatedBy: "\n")
-        guard let diffStartIndex = actualDiffStartIndex(in: lines) else {
-            let trimmedBody = normalizedBody.trimmingCharacters(in: .whitespacesAndNewlines)
-            return trimmedBody.isEmpty ? [] : [.plainText(trimmedBody)]
-        }
-
-        var blocks: [InboxMessageContentBlock] = []
-        let leadingPlainText = lines[..<diffStartIndex]
-            .joined(separator: "\n")
-            .trimmingCharacters(in: .whitespacesAndNewlines)
-        if !leadingPlainText.isEmpty {
-            blocks.append(.plainText(leadingPlainText))
-        }
-
-        let remainingLines = Array(lines[diffStartIndex...])
-        let signatureIndex = remainingLines.firstIndex(where: isEmailSignatureSeparator)
-
-        let diffLines: ArraySlice<String>
-        let trailingPlainText: String
-        if let signatureIndex {
-            diffLines = remainingLines[..<signatureIndex]
-            trailingPlainText = remainingLines[signatureIndex...]
-                .joined(separator: "\n")
-                .trimmingCharacters(in: .whitespacesAndNewlines)
-        } else {
-            diffLines = remainingLines[...]
-            trailingPlainText = ""
-        }
-
-        let diff = diffLines.joined(separator: "\n").trimmingCharacters(in: .whitespacesAndNewlines)
-        if !diff.isEmpty {
-            blocks.append(.diff(diff))
-        }
-
-        if !trailingPlainText.isEmpty {
-            blocks.append(.plainText(trailingPlainText))
-        }
-        return blocks
-    }
-
-    private nonisolated static func actualDiffStartIndex(in lines: [String]) -> Int? {
-        if let explicitDiffIndex = lines.firstIndex(where: { $0.hasPrefix("diff --git ") }) {
-            return explicitDiffIndex
-        }
-
-        for index in lines.indices {
-            let line = lines[index]
-            guard line.hasPrefix("--- ") else { continue }
-            let nextIndex = lines.index(after: index)
-            guard nextIndex < lines.endIndex else { continue }
-            let nextLine = lines[nextIndex]
-            guard nextLine.hasPrefix("+++ ") else { continue }
-
-            let oldPath = String(line.dropFirst(4))
-            let newPath = String(nextLine.dropFirst(4))
-            let looksLikeUnifiedDiff = (oldPath.hasPrefix("a/") || oldPath == "/dev/null") &&
-                (newPath.hasPrefix("b/") || newPath == "/dev/null")
-
-            if looksLikeUnifiedDiff {
-                return index
-            }
-        }
-
-        return nil
-    }
-
-    private nonisolated static func isEmailSignatureSeparator(_ line: String) -> Bool {
-        line == "-- " || line == "--"
-    }
-
-    private nonisolated static func normalizeLineEndings(in text: String) -> String {
-        text
-            .replacingOccurrences(of: "\r\n", with: "\n")
-            .replacingOccurrences(of: "\r", with: "\n")
-    }
-
     private static func stripLeadingFromLineIfPresent(in body: String) -> String {
         let lines = body.components(separatedBy: "\n")
         guard let firstLine = lines.first, firstLine.hasPrefix("From:") else {