fix: preserve Cargo context in parallel Rust resolution - #1850

Summary

• parse the root Cargo manifest once for direct parallel resolution and install the shared immutable context in each resolve worker
• run Rust cross-LSP for workspace files even when per-file resolution already emitted a confident result
• let a manifest-qualified cross-crate result supersede only the same caller, source span, kind, and source origin
• add deterministic two-worker Cargo workspace regressions plus retain the end-to-end production repro

References #56.

Root cause

The sequential cross-LSP driver built Cargo workspace context, but direct cbm_parallel_resolve workers had no manifest in their thread-local resolver state. In addition, the parallel eligibility heuristic skipped cross-LSP when a same-named local function had already produced a high-confidence result. That local result could then out-rank the correct workspace-member target.

Alternatives and trade-offs

Selected: build the manifest once on the coordinator, borrow it read-only for the worker lifetime, install it in worker TLS, and clear TLS before teardown. This preserves the existing cbm_parallel_resolve public signature, avoids repeated filesystem parsing, and matches the resolver's existing thread-local interface.

Alternatives considered:

• Add a manifest parameter to cbm_parallel_resolve. This makes ownership explicit, but expands a frozen public surface and every caller for context that can already be derived from the pipeline context.
• Parse Cargo.toml independently in every worker. This avoids sharing a pointer, but duplicates I/O and parsing and complicates per-thread ownership.
• Keep the existing confidence/eligibility heuristic. This is cheaper for the affected Rust files, but preserves the wrong-local suppression bug.
• Replace all same-named resolutions globally. This is simpler, but can conflate distinct occurrences. The selected replacement is restricted to exact caller, span, kind, and origin.

The selected fix adds a cross-LSP pass for Rust workspace files with semantic sites even when per-file results appear complete. That is bounded extra work in exchange for Cargo-qualified correctness; generated-file exclusions remain intact, and the manifest is parsed only once.

Deterministic proof

On exact main 909051c:

• RED reproduced three times: 70 controls passed and only the two new target tests failed
• final narrow suite: 72 passed
• production-only eligibility revert: 71 passed, with exactly the confident-local target failing
• production-only worker-manifest revert: 70 passed, with both target tests failing
• combined parallel and Rust-LSP filters: 594 passed
• filtered production repro: 1 passed

Validation

• make -j2 -f Makefile.cbm lint-ci
• native macOS arm64 scripts/test.sh: 7,585 passed, 0 failed, 8 documented skips across 139 suites; watchdog and security tail checks passed
• independent exact-commit review of bafe4f3: PASS, no findings
• DCO sign-off present

Platform caveat

The full local-CI run completed on macOS arm64. Linux arm64/amd64 and real-Windows local legs were not run in this isolated package; the PR remains unmerged for downstream platform CI and review.