feat(php): use new FFI for Arboricx

AGENTS.md

@@ -63,7 +63,7 @@ Arboricx is the portable executable-object format used by tricu. The project now
 | **tricu (self-hosted)** | `kernel_run_arboricx_typed.dag` | A self-hosting Arboricx parser/executor written in tricu itself. Used as a kernel inside the Zig host for maximum portability ("cool but useless" — ~3s for `append`) |
 | **Zig** | `ext/zig/` | **Production host** — native bundle parser, WHNF reducer, C ABI (`libarboricx.so` / `.a`), CLI (`tricu-zig`), Python FFI support |
 | **JavaScript (Node)** | `ext/js/` | Native bundle parser, manifest decoder, Merkle DAG verifier, Tree Calculus reducer, CLI runner |
-| **PHP** | `ext/php/` | Tree Calculus reducer, codecs, kernel loader, CLI runner |
+| **PHP** | `ext/php/` | FFI wrapper around `libarboricx.so`, CLI runner |
 
 All hosts share the same bundle format and Merkle hashing scheme.
 
@@ -240,6 +240,8 @@ The kernel path is kept as a "cool but useless" fallback — the DAG is tiny (~3
 | `packages.default` / `packages.tricu` | Haskell tricu package |
 | `packages.tricu-zig` | Zig CLI + `libarboricx.a` + `libarboricx.so` + `arboricx.h` |
 | `packages.tricu-zig-tests` | **Separate test target** — C ABI + native bundle + Python FFI tests |
+| `packages.tricu-php` | PHP source + `libarboricx.so` + `tricu-php` wrapper script |
+| `packages.tricu-php-tests` | **Separate test target** — PHP FFI tests against fixture bundles |
 | `packages.tricu-container` | Docker image |
 | `checks.default` / `checks.tricu` | Haskell test suite via Tasty/HUnit |
 
@@ -287,12 +289,9 @@ tricu/
 │   │   │   ├── codecs.js
 │   │   │   └── cli.js
 │   │   └── test/
-│   ├── php/            # PHP bundle loader + reducer
+│   ├── php/            # PHP FFI host for libarboricx.so
 │   │   ├── src/
-│   │   │   ├── functions.php
-│   │   │   ├── codecs.php
-│   │   │   ├── kernel.php
-│   │   │   └── Tree/
+│   │   │   └── ffi.php
 │   │   └── run.php
 │   └── zig/            # Zig production host
 │       ├── build.zig