[WIP] Remove php-server-child and manage native PHP child processes directly

[fredrikekelund]

Related issues

• Related to Make native-php E2E tests pass on Windows #3343. Includes the changes from that PR

How AI was used in this PR

Codex was used extensively to drive the implementation of this PR.

Proposed Changes

Important

Despite not being a draft, this PR isn't fully ready for review yet. That's because E2E tests won't trigger on CI if it's a draft.

This is a large refactor that makes it so the process manager daemon only has to deal with direct child processes. With the previous approach, php-server-child.ts would spawn PHP child processes, which became grandchildren of the process manager daemon. These are not as simple to manage as direct child processes.

With this PR, consumers tell the process manager daemon to spawn a type of process rather than a specific JS script. The process types have separate implementations that conform to a ManagedProcess interface.

Testing Instructions

TBD

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wpmobilebot]

📊 Performance Test Results

Comparing 5fbdb67 vs trunk

app-size

Metric	trunk	5fbdb67	Diff	Change
App Size (Mac)	1409.57 MB	1668.99 MB	+259.41 MB	🔴 18.4%

site-editor

Metric	trunk	5fbdb67	Diff	Change
load	1487 ms	1502 ms	+15 ms	⚪ 0.0%

site-startup

Metric	trunk	5fbdb67	Diff	Change
siteCreation	8092 ms	8088 ms	4 ms	⚪ 0.0%
siteStartup	4928 ms	4942 ms	+14 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[fredrikekelund]

I embarked on this thinking that the E2E tests on Windows were maybe having trouble killing the PHP grandchild processes. However, ultimately, that wasn't the case. Some parts of this implementation are good, but we could go even further and achieve a clearer system if we only had to deal with a single runtime type. To that end, I'll close this PR for now and revisit this in a month or so.