feat(extract): add opt-in --cargo crate dependency extractor

[giovanecesar]

Closes #1264

What / why

Graphify's Rust graph is source-AST only today. Crate-level dependencies live in
Cargo.toml, so workspace dependency layers are invisible to the graph.

This adds an opt-in --cargo extractor, mirroring the existing --postgres extractor path.
There is zero behavior change unless --cargo is passed.

This PR also includes one small watch-mode fix outside #1264. The repo requires the full
pytest tests/ gate before review, and that gate failed locally because watch-mode ignored valid
files when the temp directory had a hidden parent like .ctx-mode-*. The fix scopes hidden-path
filtering to the watched corpus root, so hidden directories inside the corpus are still ignored but
hidden parents outside the corpus no longer suppress valid file events.

Design

• Emits one node per workspace crate: crate:<name>.
• Emits directed internal crate edges with relation crate_depends_on.
• Keeps scope to workspace-internal crates only.
• Uses the raw extractor shape; build.py remains unchanged.
• Uses lazy tomllib / tomli loading with a clear Python 3.10 error.
• Does not change pyproject.toml or uv.lock.

Deferred on purpose:

• External dependency nodes (serde, tokio, etc.).
• dev-dependencies / build-dependencies.
• Renamed dependency resolution.
• Directed graph behavior changes.

Changes

• graphify/cargo_introspect.py
  • New Cargo manifest extractor.
• graphify/__main__.py
  • Adds --cargo flag, help text, usage text, extractor call, and merge wiring.
• tests/test_cargo_introspect.py
  • Adds focused Cargo manifest tests.
• graphify/watch.py
  • Fixes hidden-parent path filtering so the full local test gate can run reliably.
• tests/test_watch.py
  • Adds explicit hidden-parent coverage for the watch handler.
• README.md
  • Adds one quick-reference --cargo example.

Testing