Rust Cargo/cfg diagnostics baseline fixture

[jununfly]

This was generated by AI during triage.

What to build

Add or harden a deterministic Rust core fixture that captures the current Cargo package, crate-root, feature, and cfg diagnostic baseline for the Cargo/cfg diagnostics hardening v1 slice.

This slice is evidence-first: it should make the current behavior explicit before any diagnostic taxonomy is expanded. The source decision is docs/benchmarks/2026-06-27-rust-cargo-feature-cfg-conditional-semantics-decision.md.

Acceptance criteria

• A deterministic fixture covers repo-local Cargo package ownership, lib/bin crate roots, cfg(feature = "..."), cfg(target_os = "..."), cfg(test), and cfg_attr(...) examples.
• The fixture asserts result JSON/profile counters for the current Cargo/cfg taxonomy instead of relying on human inspection.
• The fixture documents current deferred/no-go behavior for registry dependencies and generated/build-script semantics without changing graph writes.
• Targeted Rust core tests pass.

Blocked by

None - can start immediately.

[jununfly]

This was generated by AI during triage.

Closing as implemented. Added deterministic Rust core fixture coverage for Cargo package ownership, crate roots, cfg/feature condition sources, generated/build-script no-go taxonomy, and public result JSON/profile counters.

Verification: cargo test -p zcodegraph-core rust_core_reports_cargo_cfg_condition_sources_and_suppression_diagnostics -- --nocapture; cargo test -p zcodegraph-core.