clean-house sweep: anchor drift, app/-prefix paths, D-59 sunset

Fixes 3 substring-truncated cross-reference anchors so they match
the full heading slug:
- D-3 link in architecture.md
- D-40 link in process.md (heading gained the [SUPERSEDED] tag)
- Q-15 link in questions-process.md

Strips the legacy `app/` prefix from path references in 5 files —
the dirs were flattened to repo root in the Flutter rebuild
(D-56). Three "was `app/...`" historical references in D-5 and
D-56 are deliberately preserved as record of the dissolution.

Adds an inline (tracked in T-88) note to D-59 so the
"must track dugite-native releases for security updates" intent
is wired to a backlog item — RULE-SUNSET-WITHOUT-TICKET would
otherwise keep flagging it on every sweep.

Co-Authored-By: Claude <noreply@anthropic.com>

decisions/testing.md

@@ -41,7 +41,7 @@ Test pyramid, drivers, client-side constraint.
 
 ### D-28: Test organisation — mirror `lib/` in `test/`
 - **Date:** 2026-04-21
-- **Decision:** Every test file lives at the same relative path as its subject. `app/lib/kernel/src/i18n/catalog_loader.dart` pairs with `app/test/kernel/i18n/catalog_loader_test.dart`. No separate `unit/` vs `widget/` directories; test type is detected by what the test imports.
+- **Decision:** Every test file lives at the same relative path as its subject. `lib/kernel/src/i18n/catalog_loader.dart` pairs with `test/kernel/i18n/catalog_loader_test.dart`. No separate `unit/` vs `widget/` directories; test type is detected by what the test imports.
 - **Rationale:** Matching paths makes "jump to test" predictable in any editor. Type-by-imports matches how `flutter test` already works.
 - **Cost:** Large feature folders mirror into large test folders. Acceptable.
 - **Raised by:** 2026-04-21 planning.