Standardize batch error envelopes

[Widthdom]

Summary

• Standardize batch --json-summary success and failure records on the versioned typed error envelope, including malformed input, schema/policy rejection, unsupported commands, child failures, timeouts, and cancellation.
• Keep child stdout/stderr out of failure records by default and expose bounded raw_streams only through explicit --include-raw-streams opt-in.
• Preserve deterministic input order and summaries across serial and parallel execution, including cancellation races, through a bounded per-reader input pump that retains an in-flight line for a subsequent batch invocation.

Validation

• dotnet build CodeIndex.sln -c Debug --no-restore — passed, 0 warnings.
• dotnet build CodeIndex.sln -c Release --no-restore — passed, 0 warnings.
• Issue Standardize typed error envelopes for every batch failure #4871 tests — net8.0: 8/8 passed; net9.0: 8/8 passed.
• All RunBatch tests — net8.0: 35/35 passed; net9.0: 35/35 passed.
• dotnet format CodeIndex.sln --no-restore --verify-no-changes — passed.
• dotnet run --project tools/CodeIndex.Changelog -- check — 41 fragments validated.
• Full Release suite, net8.0 — 10,781 passed, 7 skipped, 1 unrelated existing watcher failure: RunCore_SubprojectObservesAncestorIgnoreFileChanges_Issue4592; its focused retry failed in this worktree, while the isolated origin/main baseline passed.
• Full Release suite, net9.0 — 10,307 passed, 420 skipped, the same unrelated watcher failure.

Adversarial review

Two Codex adversarial-review rounds were completed. The actionable findings were addressed: cancellation now wakes blocked input subscriptions without an orphan per-invocation reader, consumed and concurrently prepared items receive ordered typed records, queued items drain before the final summary, and cancellation takes precedence over earlier failures and blank/oversized branches.

Documentation and changelog

• Updated the English and Japanese batch contracts in DEVELOPER_GUIDE.md and TESTING_GUIDE.md.
• Added bilingual fragment changelog.d/unreleased/4871.fixed.md.

Follow-up candidates

None in scope. The validation-only watcher instability is already tracked by #4592.

Fixes #4871