docs: cacheManager TTL rationale

[nanaf6203-bit]

Summary

src/lib/cacheManager.ts uses scattered TTL constants without a documented rationale. New contributors can't reason about why defaults were chosen.

Affected area

• src/lib/cacheManager.ts
• src/types/cache.ts
• docs/ (ARCHITECTURE.md not present)

Acceptance criteria

• Document every default TTL in a single table.
• Add an ARCHITECTURE.md section explaining eviction order and capacity reasoning.
• Add unit tests that lock the documented behaviour in.
• Reference the doc from each TTL constant.

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Filed automatically as part of the PropChain code-review sweep.