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.

Validation: 618 tests and 254 subtests; strict typing and repository style; real-process tests parse the whole stdout document after Python, descriptor, original-stream, and child writes and exercise native overflow.

Train: follows #419. Fixes #379.