docs: refine resource cache phase 2 design

[mahyarmlk]

Summary

Refines the existing Resource Cache and Stale Semantics proposal with a clear phased implementation plan. Phase 2 is scoped to cache.key only — no cacheTime, SWR, or cross-navigation cache.

Changes

• Updated status header to reflect Phase 1 implementation + Phase 2 design
• Added comprehensive Phase Plan section (Phases 1–6+)
• Added Approved Next Implementation Slice section answering all 10 design questions from the task
• Added Behavior Specification for Phase 2: type additions, key derivation, entry lifecycle, active key visibility, backward compatibility
• Added Test Plan with 21 unit test scenarios
• Added Compatibility Notes covering migration, feature interaction, and deferred open questions
• Updated Implementation Status table to include Phase 2 decisions

Phase 2 Scope

• cache.key: (context) => ResourceKey only
• Scoped to one runtime + one ResourceNode
• Internal Map<ResourceKey, CacheEntry> storage
• Per-key deduplication and staleTime
• No-runtime-code change (docs/proposal only)

Validation

• pnpm lint ✅
• pnpm typecheck ✅
• pnpm build ✅
• pnpm changeset status ✅ (no changes needed — docs only)