Stdlib: htmlentities() and html_entity_decode() VM + LLVM JIT/AOT

[PurHur]

Goal

Add htmlentities() and html_entity_decode() for the static web subset (issue #78 Phase 2 Stdlib).

Subset

Same entity set as htmlspecialchars() / htmlspecialchars_decode() (UTF-8, ENT_QUOTES / ENT_COMPAT flags). No full HTML translation table.

Acceptance

• VM + JIT + AOT compliance PHPT
• docs/capabilities.md regenerated
• ./script/docker-exec.sh targeted PHPUnit green

[PurHur]

Closed by #2473 — htmlentities/html_entity_decode VM + JIT + AOT with compliance and AOT PHPT.