fix: reject unreadable manifest path shapes

[johnson-ricon]

Finding

The changed-note reducer treats only exact readable-path equality as a manifest collision. A manifest can also map one blob to shared and another to shared/child.md. A full readable-tree materialization cannot represent that ref and fails regardless of row order, but the reduced path can select only the changed child ID, omit the colliding parent ID, and emit a plausible patch.

This fix validates the complete existing-blob mapping for each ref and rejects any readable path that is also another mapped path's directory. Exact duplicate paths remain supported for the alias semantics repaired by #177.

Validation

• mise run test — 405 BATS tests and 9 Python tests passed
• mise run test diff — 15/15 passed
• all eight Codebase lints passed
• mise run doctor passed; optional local pre-commit hook remains uninstalled
• bash -n lib/diff.sh .mise/tasks/diff
• readme build --check
• git diff --check

Fixes one adversarial review finding on #176.