refactor: migrate FFI layer to Lib singleton with instance methods - #25

This PR replaces the old Bindings interface + static helpers FFI pattern with a singleton Lib class that uses @method annotations and __call() forwarding for a cleaner, more idiomatic PHP developer experience.

Motivation and Context

The old architecture had a separate Bindings interface listing all ~140 C function signatures for IDE autocompletion, plus a Lib class with static helper methods. Lib::get() returned a raw \FFI typed as Bindings&\FFI, which wasn't a natural PHP API surface. All helpers were static, making the codebase harder to navigate and refactor.

What's Changed

• Lib is now a proper singleton with @method PHPDoc annotations (replacing Bindings.php, which was deleted)
• Lib::get() returns a Lib instance with full IDE autocomplete via @method
• All C function calls forward through __call() to the underlying \FFI instance
• checkStatus, getLastError, createCArray, readSizeTArray (renamed from extractShapeFromPointer) converted to instance methods
• createShapeArray and createOutputMetadataBuffers removed and inlined at all call sites
• All caller variables renamed from $ffi to $lib to reflect the Lib type
• Updated static analysis instructions in AGENTS.md and CONTRIBUTING.md

Breaking Changes

None. All public APIs remain identical; this is purely an internal refactor.