perf(cancel): compact escrow lock storage key

[Beneaththesky001]

Summary

• replace the verbose CancelLock instance-storage key with compact C
• preserve cancel-lock enforcement across normal operations and admin overrides
• add regression coverage for the compact key and locked-state behavior

Closes #296

Verification

• cargo fmt --all -- --check
• cargo test -p milestone-escrow (312 passed)

Fixes #296

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[godamongstmen897]

@Beneaththesky001 FIX FAILED CI

[Beneaththesky001]

@Goldii-locks @Beneaththesky001 Status update for #296: pushed commit 1b1a32f aligning all cancel-lock accesses with upstream CancelLock; cargo test compiled 427 tests and passed 425, with 2 existing tax-withholding failures. Please review the linked CI result and rerun checks where applicable.

[Beneaththesky001]

@Goldii-locks @Beneaththesky001 PR #358 references Fixes #296. Please review/rerun CI.