Preserve structured child errors in batch JSON summaries

[Widthdom]

Batch JSON summaries replaced actionable child errors with generic exit-code errors. Both sequential and parallel batches now preserve safe child JSON classifications and measured budget/retry fields, including E028 failures from status explanations and search arrays.

Child error parsing has a 64 KiB UTF-8 limit, depth limit, field/type checks, and diagnostic sanitization. Invalid output keeps the existing fallback; explicit capture/cancellation/timeout errors and parent output limits retain precedence. Raw failed streams remain available only through --include-raw-streams.

Validation:

• Final merged build passed with zero warnings/errors. Batch, parser, help/schema, JSON writer policy, and upstream size-policy checks: 367 passed on each of net8.0 and net9.0 (including all 51 batch/parser tests).
• Real-repository smoke checks preserve E028 and retry data for both issue reproductions.
• Formatting, diff whitespace, and changelog fragment validation passed.
• Initial local full suite: net8.0 had 12,244 passed / 4 failed / 9 skipped; net9.0 had 11,735 passed / 24 failed / 435 skipped. A dogfood index refresh overlapped that run. After the refresh finished, one focused retry covering every failed method passed all 47 selected cases on each framework. The original full run was not clean; the log-hint investigation is tracked in Investigate full-suite-only failure in global log report-hint test #5264.
• Root and workspace freshness checks passed without an environment override after integrating the latest main.
• All GitHub CI checks passed, including the Linux/Windows/macOS test lanes, net9.0, CodeQL, changelog fragments, and license policy.
• Codex adversarial review completed two rounds. The final result was: No blocking/actionable issues found.

Updated CLI help and English/Japanese README, user, developer, and testing documentation. Changelog fragment: changelog.d/unreleased/5259.fixed.md.

Follow-up references: #5258, #5264.

Fixes #5259