test(windows): ConPTY e2e coverage for the #124 parity fixes

[c1-squire-dev]

Summary

Automated end-to-end coverage for the Windows/ConPTY parity fixes in #124,
whose evidence was host-run only. The suite is #[cfg(windows)] with no
Cargo-feature gate, so the stock cargo test --workspace CI leg on
windows-latest runs it (the docker/bwrap suites stay feature-gated and
Linux-only). Six tests, one per fix:

1. run | consumer no-hang — faithful cmd.exe pipeline repro; asserts
   the consumer sees EOF within 90s (the pre-fix hang lasted until the 15-min
   daemon idle timeout). Guards the win_spawn handle-inheritance allow-list.
2. Startup DSR answered — new dsr_probe bin (src/bin/) enables VT
   input on its own console, emits ESC[6n, and prints the reply; without the
   take_pty_writes write-back it blocks forever and the sentinel never
   appears.
3. tail --follow trailing bytes — a 200-lines-then-exit-fast child;
   asserts line200 arrives before the stream ends (exit_drained /
   DRAIN_GRACE) and that the stream ends at all.
4. die descendant tree-kill — grandchild carries a per-test sentinel in
   its command line; Win32_Process verifies it is gone after die.
5. signal SIGINT — ETX via the ConPTY input stops ping -t
   (wait --exit confirms).
6. signal SIGBREAK — rejected with the actionable "cannot be delivered"
   error, not a false success.

Each test runs an isolated session + socket dir and ties the daemon to the
test process (AGENT_TUI_MONITOR_PARENT_PID) plus a best-effort
daemon shutdown on drop — parallel-safe, no orphaned daemons.

Testing

• cargo check / cargo clippy --all-targets -- -D warnings for
  x86_64-pc-windows-msvc — the suite and dsr_probe compile for Windows.
• cargo fmt --check, cargo test -p agent-tui-integration (Linux aarch64;
  the new file is cfg-stripped there).
• The tests themselves require Windows: authored on Linux, so the
  windows-latest CI leg is their first execution environment. The DSR probe
  mechanics (VT-input read of the cursor reply) are the most likely spot to
  need a tweak after a real run.

🤖 Generated with Claude Code + Squire