feature/universal runtime

[nahime0]

Summary

Implements the universal runtime callable descriptor work from the roadmap. Runtime callables now carry the metadata needed for signature/default/by-ref/variadic handling, receiver and capture environments, and descriptor-backed invocation across the supported callable shapes.

What Changed

• Added runtime callable descriptor coverage for string callbacks, callable arrays, closures, first-class callables, object __invoke, static and instance methods, builtins, and extern callable shapes.
• Routed dynamic callable execution through descriptor invokers for call_user_func(), call_user_func_array(), preg_replace_callback(), SPL iterator/callback helpers, array callbacks, fibers, and direct expression calls.
• Preserved receiver and capture environments for runtime-selected callable arrays, first-class method callables, closure aliases, branch-selected descriptors, and invokable objects.
• Normalized descriptor invocation metadata for named arguments, defaults, variadics, by-reference parameters, spread arguments, and hidden receiver/capture parameters.
• Added stateful FFI callback trampolines: descriptor-backed callables passed to extern callable parameters are retained in a per-callsite slot and exposed to C as generated ABI-compatible trampoline symbols.
• Updated docs and roadmap entries to describe the runtime callable descriptor model and the supported extern callback boundary.

FFI Callback Boundary

Extern callback descriptors now support fixed C-compatible scalar/pointer signatures: int, float, bool, ptr, and void returns, with parameters from int, float, bool, and ptr. PHP heap/value types across the C callback boundary, such as string, arrays, objects, variadics, defaults, and by-reference callback parameters, remain rejected until their ownership and lifetime rules are modeled safely.

Verification

• cargo check
• Focused FFI callback tests
• Focused extern callable error tests
• cargo build
• cargo test -- --include-ignored
• git diff --check
• Assembly comment alignment checks for touched codegen files