build: stale .map sidecar left behind when switching --source-map to --inline

[dean0x]

When a build previously used --source-map to produce an external .map file, then is re-run with --inline to embed the map, the old .map sidecar remains on disk.

Current behavior: The --inline flag does not clean up the previous external map file.

Suggested fix: Apply the existing probe_and_remove_stale pattern (used for other stale outputs) to also detect and remove stale .map sidecars when switching to --inline.

Deferred from the v0.4.0 dogfooding remediation (PR #196) — flagged during the 2026-07-17 dogfooding campaign; deliberately not implemented there pending design discussion.

[dean0x]

Already fixed in 85a4c05 (PR #183) and never closed at the time.

build.rs:1326-1347 implements stale sidecar reconciliation (AC-FUNC-10) calling verify_then_delete_map. The fix is test-pinned by sm13_inline_build_removes_stale_sidecar at cli_source_map.rs:556-583 which asserts !map_path.exists() — confirming the stale .map file is removed when --source-map switches to --inline.

Closing as fixed.