test(aot): 004-ApiJson compile+run and json_encode PHPT

[PurHur]

Summary

• Add test/fixtures/aot/cases/json_encode_api.phpt — AOT compile+run for header, http_response_code(200), and json_encode (issues Stdlib: json_encode and json_decode for API endpoints #61, Examples: 004-ApiJson — minimal JSON API endpoint (lint-first) #270).
• Extend ExampleWebAotTest with testApiJsonExampleFile for examples/004-ApiJson/example.php (matches 001/002 disk-based AOT coverage).
• Add ExamplesCompileTest::testAotExecuteSmoke004ApiJson — bin/compile.php -o link + execute smoke in the examples CI gate.
• Regenerate docs/capabilities.md (json_encode notes AOT PHPT).

Test plan

Verified locally in Docker (php-compiler:22.04-dev, harness tar-copy via ./script/docker-ci-local.sh):

LLVM 9 found at /opt/llvm9
PHPUnit VM/compliance: OK (417 tests, 768 assertions, 6 skipped)
PHPUnit serve: OK (36 tests, 364 assertions)
JIT MCJIT probe failed → @group aot only (known; malloc lookup in jit.php)
bootstrap-aot-lint: 2 target(s) OK
PHPUnit @group aot: OK (98 tests, 255 assertions)  # was 95 tests before this PR

New cases exercised:

• AotTest → json_encode_api PHPT
• ExampleWebAotTest::testApiJsonExampleFile
• ExamplesCompileTest::testAotExecuteSmoke004ApiJson

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