Fix excessive prefix-matching work in workspace backup preview and restore#188

Summary

Problem: Colon-heavy mail references cause repeated prefix slicing and lookup during backup preview and restore.

Result: Use a compressed prefix tree to scan each reference once while preserving the first listed matching account. Add synthetic regression coverage for overlapping account IDs, reordered accounts, Unicode, unknown references and colon-heavy references across typed fields.

Changelog: Update CHANGELOG.md with the security improvement and docs/KB.md with prefix-matching behavior and bounds.

Evidence

• Before: The pinned base scans every colon and slices each candidate prefix. After: The candidate traverses compressed prefix edges and retains the earliest matching account index. Runtime behavior remains unverified.

Merge risk

Rollback: Revert the prefix-tree implementation, tests and documentation. Reverting code cannot undo records already restored.

Affected scope: Account-prefixed reference remapping during workspace backup preview and import, including projects, commitments, rules, proposals and mail state. Incorrect matching could associate restored references with the wrong account.

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