[codex] Add RunDevServer SIGTERM regression check

[mpscholten]

Summary

• add a dedicated flake check that launches the real RunDevServer
• use the existing integration-test app as a real fixture with a boilerplate-style .ghci
• force the initial GHCi load to stay active long enough to SIGTERM RunDevServer mid-load and assert the child ghci process exits

Why

This is intended to reproduce the orphaned-GHCi bug from #2606 in CI. On master, RunDevServer still exits without reliably terminating the detached ghci child during the compile window, so this PR is expected to fail until the fix is applied.

User Impact

If this check fails, it means a development restart can leave a stray ghci process behind holding the app port after RunDevServer receives SIGTERM.

Root Cause

RunDevServer starts ghci in its own process group. During the initial load, terminating the parent process alone is not enough to guarantee the child process group dies too.

Validation

• bash -n integration-test/run-devserver-sigterm-check.sh
• CI should exercise the new run-devserver-sigterm flake check and fail on current master

[github-actions]

Core Size & Compile Allocations Benchmark

Metric	Baseline (master)	This PR	Change
Core size	11346666 bytes	11346661 bytes	-0.0%
Compile allocations	27762409720 bytes	27762200304 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	2.99ms	2.99ms	0.0%
p50	3.10ms	2.80ms	—
p99	5.50ms	5.50ms	—
Min	0.60ms	0.50ms	—
Max	11.70ms	54.40ms	—
Req/s	3273	3322	—

HTTP latency within threshold

Top 10 modules (this PR)

Module	Size (bytes)
Web.Types.thr	547346
Web.FrontController.thr	428416
Web.Routes.thr	423681
Web.View.Threads.Show.thr	325444
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