Cargo build-cache invalidation reads .cargo/config.toml where cargo reads .cargo/config, so it cleans the wrong target dir #1832

[agent] Filed by the scheduled architecture audit (audit-core). Register: register.

Kind: bug (with a duplication root cause). Register ID: A01.

Problem

The cargo config chain is read in two places, and the two copies prefer different files when a .cargo/ directory holds both config and config.toml:

• utils/cargo_build_cache.rs#L151-L183 ConfigDirs::read tries ["config.toml", "config"], so config.toml wins.
• formats/cargo/source_replacement.rs#L110-L142```` config_layer_paths tries `["config", "config.toml"]`. Its doc comment says this matches cargo ("cargo prefers the legacy extensionless file when both exist"). It also skips `$CARGO_HOME` when an ancestor already named that directory.

The two copies have drifted on two more points. For the $CARGO_HOME base, ConfigDirs::read falls back to unwrap_or_default() and config_layer_paths to unwrap_or(home). And only config_layer_paths checks whether an ancestor already named $CARGO_HOME.

Proof (executed on main @ b291caa). I added a scratch test in a throwaway worktree. It makes ws/.cargo/config with target-dir = "legacy" and ws/.cargo/config.toml with target-dir = "modern":

AUDIT build_dirs=["/tmp/.tmp2e21zC/ws/modern"] cargo_layers=[("/tmp/.tmp2e21zC/ws/.cargo/config", "/tmp/.tmp2e21zC/ws")]

Cargo builds into legacy/. So invalidate_project (called from apply and rollback) removes fingerprints under modern/, and the stale fingerprints that cargo actually uses survive. The next cargo build then links the unpatched code, which is the failure this module exists to prevent.

Proposed change

• Build ConfigDirs::read on config_layer_paths(cwd, cargo_home, Path::is_file). Keep only the [build] target-dir/build-dir extraction, and delete its own ancestor walk and file pick. This is about −15 lines.
• Optionally, move config_layer_paths to a neutral home such as formats/cargo/config.rs, since it is now shared by the source-replacement reader and the build-cache cleaner.

Size: 1–2 files, about +5/−20.

Acceptance criteria

• A unit test in cargo_build_cache.rs covers the case above: both files exist, and the build dirs resolve to the extensionless file's target-dir.
• build_dirs_honours_env_config_and_workspace_root still passes.
• The source-replacement tests are unchanged.