Stabilize child stdio lifecycle tests

[beiwei30]

Summary

• split normal lifecycle grace, explicit escalation timing, and the outer deadlock watchdog so parallel scheduler delay cannot turn graceful exits into forced termination
• add a deterministic two-phase broken-stdin fixture handshake while preserving exact StdinIo, pending-request release, and child-reaping assertions
• document the reproduced causal orderings and closeout evidence; production transport code and the maintenance spawn disposition remain unchanged

Verification

• cargo test -p orbcode-app-server-client --test child_stdio_transport
• cargo test -p orbcode-app-server-client --test child_stdio_transport -- --test-threads=1
• exact canonical initialize, broken-stdin, and outbound-payload tests
• 20 consecutive default-parallel child_stdio_transport target runs
• cargo test -p orbcode-app-server-client
• cargo test -p orbcode --test child_stdio_client_e2e
• maintenance self-test and audit, docs audit, rustfmt, focused clippy, and focused check
• two consecutive complete scripts/check.sh runs from a clean worktree

No retries, ignored tests, serial annotations, public API changes, protocol changes, or production Rust changes were introduced.

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