feat: PHP eval() support via the Magician interpreter

[nahime0]

Summary

This PR adds full eval() support to elephc through Magician (crates/elephc-magician), a PHP interpreter that ships as an optional static library. Programs that never call eval() are completely unaffected: the interpreter is linked only when eval is actually used, and native-only binaries keep their current size and dependencies.

1,207 commits · 2,128 files · +268k/−11.5k lines

How eval is executed

Eval sites are compiled with a three-tier strategy, picked per call site by coordinated classifiers in the plan, lowering, and codegen layers:

1. Literal AOT — string-literal fragments that the compiler can prove static (scalar expressions, static builtin calls, array literal reads, switch/match/ternary, user-function calls, …) are compiled ahead of time into native EIR functions. No interpreter, no bridge, no runtime cost beyond a direct call.
2. Scope AOT — fragments that read or write caller locals but are otherwise static run as native functions with direct read parameters or scope write-back, still without linking the interpreter.
3. Bridge — everything else (dynamic code, declarations, object semantics) executes in the Magician interpreter through the __elephc_eval_* FFI bridge, with bidirectional scope synchronization between native locals and the eval scope.

Native ↔ eval interop

• Caller locals are synchronized across the eval boundary in both directions, including type-changing writes (scalar slots widen to boxed Mixed storage) and container values.
• eval'd code can call AOT functions, methods, and constructors with full PHP visibility semantics: private/protected access checks by calling scope, private-method shadowing over child overrides, magic __call/__callStatic fallbacks, and late static binding.
• eval-declared functions, classes, closures, and constants are callable from subsequent eval fragments and observable through the dynamic probes (function_exists, defined, reflection, …).

The Magician interpreter

• 415+ PHP builtins implemented through a declarative eval_builtin! registry (inventory-collected, one home file per builtin), covering strings, arrays, math, regex (PCRE + mb_ereg), JSON, filesystem, date/time, SPL, and reflection.
• Static-vs-eval builtin parity is enforced by dedicated parity gates (tests/builtin_parity_tests.rs).

Documentation

• New docs/php/eval.md covering supported semantics and the AOT tiers.
• The generated builtin docs now render a dual support matrix: every builtin page and index shows whether it is available in compiled (AOT) code and inside eval(), with links to both implementations. The exporter reads both registries, so the matrix cannot drift from the parity tests.

Compiler improvements landed along the way

• PHP 8.3 auto-key semantics for array literals (bool/float/numeric-string and leading-negative keys).
• array_key_exists() over boxed Mixed containers with runtime tag dispatch; float key normalization.
• is_array/is_integer dispatchable as runtime string callbacks.
• ReflectionParameter promoted-parameter metadata, newInstanceArgs() with indexed argument lists, attribute array-literal arguments.
• First-class-callable fixes (by-ref builtin params, magic fallbacks).

Testing

• 935 eval codegen tests plus 7 dedicated eval test suites (closures, callables, reflection invocation, …), all green on macOS.
• 928 Magician unit tests; full codegen/error/parser/lexer suites green.
• Branch is rebased on latest main (0.26.1 + declare support) with the generated docs byte-identical to CI regeneration.

[nahime0]

This is almost ready for a first merge. It's still experimental, but it's a good candidate for the next release