fix: skip scanner symlink cycles and map compile_batch panics

[undivisible]

Summary

Independent of #1 (load/compile hardening).

• scan_directory and LibraryScanner skip symlinks so directory cycles cannot hang discovery
• scanner also skips target, .git, build, and other non-source trees (matching detector)
• compile_batch maps worker panics to CompileError instead of join().unwrap()

Test plan

• cargo test --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• new tests: symlink skip, skip target/ headers

---

Note

Low Risk
Localized filesystem-walk and error-handling changes with tests; no auth, data, or API contract breaks beyond surfacing panics as errors.

Overview
Hardens directory discovery and parallel compilation so bad filesystem layouts and worker failures do not crash the process.

Discovery (scan_directory, LibraryScanner) now classifies entries with file_type() and ignores symlinks, so symlinked directory trees cannot cause infinite recursion or hangs. The library header scanner also skips the same non-source directories as the detector (target, .git, build, dist, zig-cache, nimcache, etc.) when recursing.

compile_batch no longer panics if a scoped worker thread panics: join() failures are turned into CompileError::Io with a clear message instead of propagating an unwrap panic.

Tests cover symlinked-dir skipping (Unix) and ignoring headers under target/.

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(requestId: serverGenReqId_5f32ba52-8171-41b1-b37c-b98aa8a43a74)

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