[codex] Fix RunDevServer SIGTERM orphaned GHCi

[mpscholten]

Summary

• add a dedicated flake check that reproduces the RunDevServer SIGTERM orphaned-GHCi bug
• install a process-local SIGTERM handler in RunDevServer
• terminate the active GHCi child process group before exiting so the dev server does not leak an orphaned compiler process

Why

This keeps the new regression check in-tree while fixing the underlying bug, so the same CI path that fails on the regression-only branch should pass here.

User Impact

Stopping the dev server during the GHCi load/compile phase should no longer leave a stray ghci process behind holding :8000.

Root Cause

RunDevServer received SIGTERM, but the active GHCi child was started in its own process group. During the load phase, exiting the parent process was not enough to reliably kill that detached child.

Validation

• bash -n integration-test/run-devserver-sigterm-check.sh
• CI should exercise the new run-devserver-sigterm flake check and pass with the fix included

[github-actions]

Core Size & Compile Allocations Benchmark

Metric	Baseline (master)	This PR	Change
Core size	11346666 bytes	11346659 bytes	-0.0%
Compile allocations	27762080712 bytes	27762083712 bytes	0.0%

Core size within threshold
Compile allocations within threshold

HTTP Latency (GET /, 5000 reqs, 10 concurrent)

Metric	Baseline (master)	This PR	Change
Mean	3.07ms	2.67ms	-13.0%
p50	3.10ms	2.60ms	—
p99	4.80ms	5.50ms	—
Min	0.60ms	0.50ms	—
Max	15.60ms	13.80ms	—
Req/s	3242	3587	—

Improvement: Mean latency decreased -13.0%

Top 10 modules (this PR)

Module	Size (bytes)
Web.Types.thr	547347
Web.FrontController.thr	428416
Web.Routes.thr	423680
Web.View.Threads.Show.thr	325442
Web.Controller.Threads.thr	322977
Web.Controller.Comments.thr	315692
Web.Controller.Users.thr	284459
Admin.FrontController.thr	276526
Admin.Types.thr	264263
build.Generated.User.thr	259919