feat(context): add CPG-01 pure context projection planner

[Wibias]

Summary

• Start CPG-01 of cache-preserving context projection with a test-first contract for the pure request-local projector/registry.
• Keep canonical Responses history authoritative and require duplicate projection to preserve an earlier fully visible exact source.
• Harden cancellation polling so mixed UTF-16 widths cannot skip abort checks.
• Make duplicate receipts identify the exact source tool and occurrence when call ids repeat.
• Cover a 10 MiB one-line tool result without canonical mutation.

Deep-review hardening

The hardening landed tests-first. The focused projector suite and full TypeScript typecheck passed before the clean branch history was restored.

The projector still fails open on scan-budget/cancellation boundaries, uses SHA-256 only for candidate bucketing plus strict string equality, and never mutates canonical context.

Verification

• Original CPG-01 TDD contract remains before implementation.
• Deep-review TDD commit: 6ac9642c7deb3c2a484d1f2ebaddf7a00714232f.
• Hardened implementation head: 7d1523150d85986cfe7e29649df17386007ce2b5.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• No user-visible live behaviour is wired in CPG-01.
• Security-sensitive changes were reviewed for bounded scans, cancellation, identity, and canonical immutability.

[github-actions]

✅ Deterministic PR hygiene checks passed.

[coderabbitai]

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