Fix import path boundaries and reuse absolute resolutions

[TristanKruse]

Summary

• Fix an existing path-boundary bug: when the project root is .../app, a resolved sibling under .../app2 must be external. Previously startswith(project_root) classified it as internal, and the graph could drop the dependency entirely.
• Cache absolute-import resolutions within an extraction session. Relative imports remain source-specific and uncached. clear_graph_cache() and CheckOptions(clear_cache=True) invalidate the session and its resolution cache.

This PR is stacked on #61. It does not change the parser or the public rule API.

Correctness

• New tests cover sibling modules and packages, repeated absolute imports, explicit invalidation when a missing target appears, and equal-spelling relative imports from different directories.
• 331 tests pass on Python 3.12 and 3.13. Ruff, strict Mypy, and git diff --check pass.
• The existing 5,000-module clean, forbidden-edge, and cycle gates pass; ten-rule violation count remains 7,494. An eight-pair real-source check preserves all 385 ordered graph edges and import kinds exactly.

Performance (paired runs on one Windows host)

• For 1,251 selected files and 3,747 imports, repeated absolute resolution reduces os.path.isfile probes from 3,747 to 2,499.
• Library-only first rule: median 0.431 → 0.406 s (8 pairs). Ten subsequent rules: 1.160 → 1.009 s (8 pairs).
• Full graph of this repository's source: 0.170 → 0.125 s (8 pairs, startup excluded).
• Fresh pytest process, 5,000 modules: 1.186 → 1.183 s (12 pairs), effectively unchanged at end-to-end scale. Small fresh-process medians improved in a separate five-pair check but are too noisy for a strong claim.

These measurements do not establish that ArchUnitPython is the fastest tool. In a separate same-session five-round end-to-end comparison on the same corpus, ArchUnitPython via pytest was 1.269 s, Tach 1.053 s, and Import Linter 0.789 s. That comparison is local diagnostic evidence, not a CI threshold.

The resolution cache assumes a stable project snapshot during a session, like the existing graph and file-inventory caches. Callers changing files should clear the cache before rechecking.