fix(rust): respect member and nested-worktree scan boundaries

[AashrayAnand]

Summary

• Stop Cargo workspace discovery at the nearest workspace, so a worktree nested beneath another checkout cannot resolve back into the outer checkout.
• Preserve all targets/features and diagnostic flags while limiting member scans to their enclosing Cargo manifest; workspace-root scans remain workspace-wide.
• Restrict rustdoc package selection to the scanned member or subtree.
• Parse Cargo metadata JSON from stdout without appending warnings emitted on stderr.

Validation

• All 140 Rust language tests pass.
• Added synthetic regressions for nested worktrees, paths containing spaces, member versus workspace selection, metadata warnings, and explicit metadata failures.

No detector is disabled and no findings are suppressed.