Add IPv6 loopback hosts entry for custom domains to fix slow .local DNS on macOS

[youknowriad]

Related issues

• Fixes .local DNS resolution slow on MacOS #3655

How AI was used in this PR

Claude Code investigated the reported .local slowdown, located the hosts-file logic, implemented the IPv6 entry plus the migration path for existing sites, and wrote the tests. The author reviewed the diff and reasoning.

Proposed Changes

On macOS, resolving a .local custom domain that only has an IPv4 entry in the hosts file can stall on an IPv6 (AAAA) lookup that has to time out before the IPv4 result is used — adding several seconds to every request for affected users (see #3655). This writes a matching ::1 loopback entry alongside the existing 127.0.0.1 entry for each custom domain, which is the standard remedy for this behavior.

Users with existing custom-domain sites are migrated automatically: the IPv6 entry is added the next time the site starts. Already-correct entries are left untouched, so no spurious elevated-permission prompt appears on startup.

The slowdown is environment-dependent (the linked issue tags it as affecting "Some (< 50%)" of users), but the extra entry is harmless for unaffected users and is the cure for those who hit it.

Testing Instructions

On a macOS machine that exhibits the slowdown:

1. Create a Studio site with a custom .local domain.
2. Measure: time curl -sI http://your-domain.local/ > /dev/null — note the time.
3. With this change, restart the site so the hosts file is rewritten.
4. Confirm /etc/hosts now contains both 127.0.0.1 your-domain.local and ::1 your-domain.local inside the # BEGIN/END WordPress Studio block.
5. Re-measure with the same curl command — the time should drop substantially.
6. Delete the site and confirm both entries are removed (no orphaned ::1 line).

Pre-merge Checklist

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

[wpmobilebot]

📊 Performance Test Results

Comparing 5480e44 vs trunk

app-size

Metric	trunk	5480e44	Diff	Change
App Size (Mac)	2341.83 MB	2341.83 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	5480e44	Diff	Change
load	788 ms	1088 ms	+300 ms	🔴 38.1%

site-startup

Metric	trunk	5480e44	Diff	Change
siteCreation	6486 ms	6483 ms	3 ms	⚪ 0.0%
siteStartup	6979 ms	6983 ms	+4 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

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

[wojtekn]

I couldn't notice any difference either, but the fix seems reasonable and harmless.

[youknowriad]

I noticed local does the same as well.