test: establish unit-test net for the deterministic core (v5 step 1)

Stands up the DomainDigTests unit-test target the v5.0.0 roadmap flags as the
mandatory first move before decomposing the god-files and locking external
contracts. The project had no XCTest unit target — only the DomainDigUITests
accessibility suite.

- New DomainDigTests target (unit_test_bundle), hosted by the app so
  @testable import DomainDig links. Mirrors the UITests build settings
  (SWIFT_VERSION 5.0 + MainActor default isolation) to avoid the XCTest
  override-isolation issue recorded for the Swift 6 targets. Wired into the
  shared DomainDig scheme's Test action, so it runs in CI and the pre-push
  audit hook automatically (both invoke the whole scheme).

- SnapshotFixture builds the deep LookupSnapshot/DomainReport models through
  their real initializer and the DomainReportBuilder, exposing only the fields
  the tests vary.

- 36 characterization tests across the four deterministic units the roadmap
  names:
  - DiffService: change classification, case/whitespace normalization, DNS
    record reorder-vs-change, summary phrasing, resolver context note,
    certificate-warning thresholds.
  - DomainReportBuilder: snapshot -> report field mapping, primary-IP and
    DNSSEC derivation, TLS status, partial-snapshot/validation passthrough.
  - DomainReportExporter: format dispatch, JSON round-trip (the machine
    contract), CSV/markdown/text/PDF structural invariants, timeline.
  - DomainDataPortabilityService: CSV round-trip and the merge/dedup semantics
    (case-insensitive collapse, OR-merged pin state, min-created/max-updated,
    recency sort) via an ephemeral UserDefaults suite.

- build.yml comment updated to reflect the second test target.

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