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- D-40 link in process.md (heading gained the [SUPERSEDED] tag)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 Three "was `app/...`" historical references in D-5 and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/accessibility.md

@@ -8,7 +8,7 @@ A11y + i18n are Tier-0 contracts, not Tier-6 polish.
 - **Date:** 2026-04-21
 - **Decision:** Every widget primitive wraps its interaction surface in a `Semantics` node at the point of creation. A11y coverage is a test-time gate (`ci/test_a11y.sh`), not a post-hoc polish pass. `ensureSemantics()` fires at app boot; Flutter's semantics tree is always populated.
 - **Rationale:** Retrofitting a11y onto a grown UI is what every project that skips this promises to do later and then doesn't. Making it a Tier-0 contract costs one `Semantics` line per primitive and a semantic-coverage test; postponing costs a rewrite.
-- **Cost:** Widget authors maintain correct labels; tests reject new primitives without semantics. Enforced by `app/test/a11y/` coverage tests.
+- **Cost:** Widget authors maintain correct labels; tests reject new primitives without semantics. Enforced by `test/a11y/` coverage tests.
 - **Raised by:** 2026-04-21 planning.
 
 ### D-21: i18n is a Tier-0 contract (fframe pattern + locale-fallback chain)