feat: versioned store-migration policy for persisted data (v5 step 2)

Third v5.0.0 roadmap item: define and implement a migration policy for the
on-device persisted store (tracked domains, history/snapshots, audits,
workflows, monitoring, settings), so data upgrades cleanly across app versions
instead of relying on a one-shot marker.

- DataMigrationService is reworked from a single boolean marker
  (`data.migrations.v3_4_0`) into a versioned runner keyed by an integer store
  schema version (`data.storeSchemaVersion`). It runs each step once in
  ascending order up to `currentStoreSchemaVersion`, stamping the version as it
  goes. Adding a future migration is now a `case N:` plus a version bump.

  Policy guarantees, all covered by tests:
  - Forward-only and idempotent; every step must be safe on an empty/older store.
  - Never downgrades: a store written by a newer build (higher version) is left
    byte-for-byte untouched.
  - Pre-versioning installs are handled: a set legacy boolean marker reads as
    "already at v1", so the v1 normalization never re-runs for them.

  v1 is the existing normalization pass (dedup + drop the legacy `watchedDomains`
  key + sanitize monitoring settings), now expressed as migration step 1.

- Docs/data-migration.md documents the persisted surface, the two independent
  version lines (store vs. backup export), when to use lenient decoding vs. a
  migration step, the runner contract, an "adding a migration" checklist, and
  backup-import compatibility. Linked from the README.

- DataMigrationServiceTests: 6 tests over legacy fixtures — fresh-store stamping,
  legacy `watchedDomains` migration + key drop, in-place dedup of the stored
  blob, idempotence, legacy-marker-as-v1, and the no-downgrade guard. Full unit
  suite: 58 passing.

1 files changed, 4 insertions, 0 deletions

diff --git a/DomainDig.xcodeproj/project.pbxproj b/DomainDig.xcodeproj/project.pbxproj
index e65c63a..4bab46e 100644
--- a/DomainDig.xcodeproj/project.pbxproj
+++ b/DomainDig.xcodeproj/project.pbxproj
@@ -10,6 +10,7 @@
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@@ -75,6 +76,7 @@
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 		15A25DF2B8BB52589D49986B /* DiffServiceTests.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = DiffServiceTests.swift; sourceTree = "<group>"; };
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@@ -229,6 +231,7 @@
 				54D7C7D97A4006831572F468 /* DomainReportExporterTests.swift */,
 				CC948A02EC0184228BC4630E /* DomainDataPortabilityServiceTests.swift */,
 				0D85A44C5F315A1644AC9073 /* LocalAPIContractTests.swift */,
+				435AFB3F99D579D3D7B58279 /* DataMigrationServiceTests.swift */,
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 			name = DomainDigTests;
 			path = DomainDigTests;
@@ -487,6 +490,7 @@
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 				A5AF921BC1C2E6E940CC05DC /* SnapshotFixture.swift in Sources */,
 				EA12012CDB4C21ABB4217D86 /* LocalAPIContractTests.swift in Sources */,
+				4F96CEB875EC3501E784CE39 /* DataMigrationServiceTests.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
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