perf: linearize manifest component expansion

[johnson-ricon]

Finding

PR #177's fixed-point component expansion rescans every manifest edge until no ID or path is added. Edge order can make a long alias chain advance only one hop per pass, so this stage is quadratic in manifest size. A 3,001-ID reverse-ordered synthetic component took 7.258 seconds at exact reviewed head 3de63a2, despite the PR's scaling goal.

This fix builds the same bipartite ID/path components with union-find and then selects all IDs whose component contains a seed. The same 3,001-ID case returns all 3,001 IDs in 0.096 seconds, with no new subprocesses or blob materialization.

Validation

• before/after synthetic 3,001-ID component: 7.258s → 0.096s
• mise run test — 404 BATS tests and 9 Python tests passed
• mise run test diff — 14/14 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.