refactor(i18n): tighten shared terms and dynamic key proofs

[limityan]

Refs #959

Summary

• PR A: move the installer titlebar default to the canonical shared.product.name term and remove the duplicated titlebar.default locale keys.
• PR B: require dynamic i18n allowlist entries to include source-level proof markers, validate those markers in i18n:audit, and document the rule.
• PR C: add explicit dynamic-key proofState reporting so current dynamic contracts are distinguishable as source-proven, not merely allowlisted.
• PR D: remove the remaining high-confidence shared-term copies for backend app-name and relay static Remote Control while preserving runtime compatibility and self-contained loading.
• Update durable i18n architecture/development docs plus concise AGENTS/CONTRIBUTING guidance; no process-plan docs are included.
• Strict review follow-up: added a regression test proving stale relay static $shared references fail i18n:audit instead of silently degrading.

Key Comparison

Metric	Before	After	Delta
Shared-term duplicate candidates	214	206	-8
Installer shared-term duplicates	3	0	-3
Core shared-term duplicates	18	15	-3
Relay static homepage shared-term duplicates	2	0	-2
product.name duplicate candidates	6	0	-6
features.remoteControl duplicate candidates	2	0	-2
Dynamic key candidates	194	194	0
Dynamic candidates with source references	0	194	+194
Dynamic candidates marked source-proven	0	194	+194

Current report also keeps confirmedUnusedKeys at 0 and l10nQualityCandidates at 1093; this PR does not attempt translation-quality cleanup.

Risks / Contract Changes

• Backend callers of legacy app-name continue to resolve through an explicit alias to shared.product.name; the copied Fluent key is removed.
• Relay static homepage now resolves { "$shared": "features.remoteControl" } through a tiny generated i18n.shared.json; it remains self-contained and does not import Web UI locale catalogs.
• Dynamic key allowlist entries are stricter: moving or renaming a key factory must update sourceReferences, and unproved contracts fail i18n:audit.
• The dynamic key count is intentionally unchanged; the improvement is proof quality and reviewability.

Verification

• pnpm run fmt:rs
• pnpm run i18n:generate -- --check
• pnpm run i18n:audit -- --report-json scripts/.tmp-i18n-pr-cd-report.json
• pnpm run i18n:contract:test (31 tests)
• pnpm --dir BitFun-Installer run type-check
• cargo test -p bitfun-core i18n -- --nocapture
• pnpm run check:repo-hygiene
• git diff --check
• adversarial review: verified no process docs/temp reports are staged, dynamic findings are all source-proven, removed core/relay shared-term duplicate classes are absent from the audit report, and stale relay $shared references are audit-blocked.