Source-map test-coverage gaps (TEST-4, TEST-5, TEST-6)

[dean0x]

Follow-up test-coverage gaps for source-map behavior, deferred from the PR #183 review. Each is incremental coverage over already-passing paths.

Items

• TEST-4 — crates/mds-python/tests — the Python check test never decodes the VLQ mappings; add a test that decodes and asserts segment values (parity with the JS VLQ self-test).
• TEST-5 — crates/mds-cli/tests — no integration test for the stdout guard; add a test covering the source-map stdout-guard path.
  → LANDED in PR fix: v0.4.0 remediation — dogfooding blockers, bug batch, UX polish, docs sweep #196 (3aef465) and PR fix: v0.4.0 dogfooding remediation — block-span lint fixes, span attribution, CLI/config UX, binding alignment, docs (#172 batch) #228 (f06a2d1). The guard is the three-case ladder at crates/mds-cli/src/build.rs:1228-1240 (stdin) and :1351-1378 (file). Covering tests in crates/mds-cli/tests/cli_source_map.rs: :1350 sm18_explicit_sidecar_flag_without_output_hard_errors (Case 2 hard error, build.rs:1355-1361); :1390 sm19_config_source_map_sidecar_without_output_degrades_with_warning (Case 3 degrade, build.rs:1364-1376); :1437 sm19q_config_source_map_without_output_quiet_no_warning; :1072 sm16a_file_inline_stdout_no_absolute_paths and :1140 sm16b_stdin_inline_stdout_now_allowed (--inline -o -, build.rs:1215-1222). None existed at PR feat: Source Map v3 across all surfaces — napi, WASM, Python, universal JS (#62) #183 — 85a4c05 had 21 tests, only sm14/15/16/17; main has 30.
• TEST-6 — crates/mds-core/tests — the segment cap is only exercised via the text @for path; add coverage for the segment cap on other paths.

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Deferred from code review of PR #183 (#62); see resolution-summary 2026-07-16_1241.

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References verified against main at 6d41777 (2026-08-26). Line numbers corrected; scope and acceptance criteria unchanged.