fix(analysis): discover external types with visibility, attributes, and generics

[thwbh]

The external-crate lookup introduced in #78 used a raw substring match (content.contains("struct X") / content.contains("enum X")), which produced both false negatives and false positives:

• false negatives for multi-line declarations and any form a stricter keyword-anchored regex would miss
• false positives where a longer identifier (e.g. ExternalFooBar) matched a search for a shorter one (e.g. ExternalFoo)

Replace the substring heuristic with a syn-based AST match on the item identifier. A cheap contains(type_name) pre-filter still skips the vast majority of registry files before paying for a parse; candidate files are then parsed with syn::parse_file and checked for a struct/enum item whose ident exactly equals the requested type name. This transparently handles pub, pub(crate), #[derive(...)] attributes, generics, and multi-line declarations, and recurses into inline mod blocks.

Files that fail to parse (macro-heavy/generated registry sources) are skipped instead of aborting the walk.

Adds 9 regression tests covering pub(crate) visibility, derive attributes, generics, multi-line declarations, pub enum with attributes, nested modules, prefix false-positive avoidance, missing types, and unparseable files. Tests run #[serial] since they share the CARGO_HOME env var.

[chrome-grid]

🟪🔷 netrunner :: intrusion report

░▒▓ N E T R U N N E R ▓▒░
   🟪 GRID STATUS: JACKED IN   |   netrunner-review: pass

CI passed cleanly. The PR replaces a fragile substring heuristic with an AST-based search for external types, correctly handling visibility, attributes, generics, and multi-line declarations. All new tests pass, including regressions for issue #82.

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📡 NETWATCH · gate checks

➖ automerge — skipped
✅ Test Suite — success
✅ Code Coverage — success
✅ Validate Examples — success

🟢 VERDICT // JACKED IN — deck's clean, netrunner-review gate open.

"Your chrome’s polished, choom—no ICE in sight. Arasaka’s gonna hate this one."
— netrunner 🟪🔷

_{verdict: APPROVE · model: mistral-medium-2508}