fix: capture child and descriptor stdout in JSON envelopes

[codeforester]

JSON invocations now capture process descriptor 1 as well as Python stdout, so inherited subprocess output remains inside the single envelope. A concurrent drain avoids pipe deadlocks, the native path enforces the JSON capture limit, and descriptor restoration precedes envelope emission.

The guides state the boundaries: wait for children, flush native stdio before returning, and use NDJSON for large output. A child retaining stdout causes a bounded capture error.

Fixes #379.

Branch maintenance

Refs #426. Targets the branch for #419. Retarget and refresh after that parent is squash-merged; preserve the ordered stack.

The branch was refreshed without rewriting history to include main at a576cc279739eae5e4cfc33ffab2a7fb56de24de.

Current-head validation

At 0441cec3b3098425d8c87aa9c8eaeee6e5496969: uv lock freshness and baseline, runtime, strict typing, style, and contracts passed locally with all declared extras. Runtime result: 629 passed, 1 warning, 262 subtests passed in 9.37s.

Hosted checks: 7/7 required checks passed; 0 checks pending; 0 unsuccessful checks at 2026-10-04T14:19:40.760978+00:00. See the PR Checks tab and #426 for subsequent results.

[codeforester]

Re-verified at 40d9e2a: addressed ✅. A detached child that still holds stdout is still an error, but it is now an explicit, typed capture_incomplete (StdoutCaptureIncompleteError) with everything drained before the timeout preserved in the single envelope, plus DEVNULL/start_new_session guidance. That meets #379's 'do not silently truncate or discard child-process output' non-goal, and keeping the error rather than returning success is a reasonable contract choice. The process-wide fd-1 note is documented. Full suite passes at the tip. The threads can be resolved.

[codeforester]

Resolved the merge conflicts by merging current main, including merged PRs #418 and #419, into this branch in commit b94249c. Preserved the #379 JSON descriptor-capture implementation and documentation, current main logging sidecar recovery, and the combined hot-path tests. Local validation passed: descriptor/JSON focused tests, full pytest, Ruff check and format, strict mypy, and documentation validation. GitHub reports the PR as conflict-free; hosted checks are running on the new head. The PR remains open and unmerged.