fix(coverage): fail fast on dead test process; harden web validation

Red team review of this PR surfaced a hang, a silent-data-loss
regression, and several validation gaps.

- Fail immediately when the test process exits before reporting a VM
  service URI. Deleting _dartRun removed the only thing that surfaced a
  dead child, so a bad --test path (or unresolvable deps, or a broken
  pubspec) hung forever instead of failing in ~1s. The child's exit code
  is now propagated.
- Throw a FormatException from HitMap.parseFiles for files that are
  neither a {"coverage": [...]} report nor a raw V8 list. The pattern
  matching switch had no default arm, so 13 of 16 malformed inputs
  silently produced an empty hitmap where they previously threw.
- Reject --branch-coverage with a web platform (it is silently ignored
  there), matching the --function-coverage treatment.
- Reject platform flags passed through to the test script: '-p chrome --
  -p vm' previously ran both platforms, bypassing validation entirely.
- Warn instead of silently writing an empty report when no coverage data
  survives filtering, which happens naturally when the web compiler
  inlines library code into test files.
- Document in --help and the CHANGELOG that web coverage is measured on
  compiled JavaScript and is not comparable to VM coverage, so
  --fail-under thresholds must be re-tuned per platform.
- Add regression tests for all of the above, plus tests contributed by
  the review for exit-code propagation, platform validation messages,
  --platform vm routing, and the V8 parsing paths.