ttl: split cache logging into explicit read/write hit/miss/corruption events

[kdy1]

Summary

Cache logging is currently summarized via a single event path, making read/write lifecycle and corruption transitions harder to observe.

Evidence

Contract Evidence:

• docs/project-ttl.md:182
• cmds/ttlc/internal/compiler/service.go:500

Current Gap

Cache logging is currently summarized via a single event path, making read/write lifecycle and corruption transitions harder to observe.

Proposed Scope

• Not documented yet.

Acceptance Criteria

• Split cache observability into explicit read and write event families.
• Represent hit/miss/corruption transitions as distinct log events.
• Preserve and populate structured fields across all cache events.
• Add tests asserting expected cache event taxonomy.

Test Scenarios

• Cache read miss path emits dedicated miss event.
• Cache read corruption path emits corruption event.
• Cache write success/failure paths emit corresponding write events.

Out of Scope

• Changing cache-key invalidation rules.
• Replacing slog with another logging framework.