fix(extract): default the AST cache location to CWD, not the analyzed source tree (#1774)

[SimiSips]

Fixes #1774.

Problem

extract() called without cache_root derives the cache root from the inferred common parent of the input files — i.e. the analyzed source tree. Analyzing a corpus outside CWD (a read-only knowledge base, someone else's checkout) silently creates graphify-out/cache/ inside that corpus, defeating output isolation via CWD. Reproduced on current v8 (0.9.13). The CLI and watcher are unaffected — they already pass cache_root explicitly (added in 01fb51b for #350) — so this only bites library callers and python -m graphify.extract.

Fix

One line: effective_root (the cache location) now defaults to Path('.') when cache_root is None, matching cache_dir()'s own default. An explicit cache_root still wins.

Deliberately not the patch suggested in the issue (defaulting root itself to CWD): root also anchors node-id relativization (path.relative_to(root)) and _augment_symbol_resolution_edges(), and pointing it at CWD would degrade ids for corpora outside CWD. Only the cache side changes; id/symbol anchoring is untouched.

Notes

• file_hash() keys include the path relative to the cache root, so existing source-tree caches of library callers go cold once and re-extract (CLI/watch caches are unaffected). No correctness impact.
• Cache entries for out-of-root files keep absolute source_file paths — an already-handled pass-through in _relativize_source_files_in / _absolutize_source_files_in.

Tests

• New tests/test_extract_cache_location.py: default cache lands in CWD and the corpus stays untouched; the CWD cache round-trips on a second run; explicit cache_root still wins.
• Full suite: uv run pytest tests/ -q — 3162 passed, 3 skipped on macOS (Python 3.12).