Fix AOT ?? on $_GET and web PHPT expectations

[PurHur]

Summary

• Fix LLVM/AOT lowering for ?? when the merge block re-initialized hoisted php-cfg temps, wiping branch assignments (empty echo for $_GET['name'] ?? 'Guest').
• Teach bin/compile.php to fall back to QUERY_STRING / REQUEST_BODY from the environment when -q/-p are not passed (matches PHPT/AotTest compile env).
• Align AOT PHPTs: coalesce_get_present exercises direct echo $_GET['name'] ?? …; in_array_router_404 expects Status: 404 (runtime header form).

Test plan

Docker (php-compiler:22.04-dev, repo copied via tar):

vendor/bin/phpunit --group aot-link --exclude-group serve
# OK (123 tests, 672 assertions)

Manual:

php bin/compile.php -q name=Ada -o /tmp/coal /tmp/t.php   # echo $_GET['name'] ?? 'Guest'
/tmp/coal   # Ada

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