feat: JS delegates with value-type parameters and return values

[triniwiz]

JS functions passed as WinRT delegates (NSWinRT.asDelegate and delegate constructors) now work when the delegate's Invoke takes floating-point values or returns a value.

Problem

Every JS delegate shared one vtable whose Invoke takes up to three pointer-sized arguments and returns nothing. That covers event handlers ((sender, args)), but not a delegate such as Int64 F(Single, Single, Single, Single):

• On x64 and ARM64, float arguments travel in vector registers, which an integer-typed Invoke never reads, so the JS function received garbage.
• The return value goes through a trailing out-pointer, which was never written, so the native caller got whatever was in that slot.

Change

• New vtable: delegates whose Invoke needs it get their own vtable. Its Invoke is a libffi closure built from the exact signature in metadata (runtime/src/delegate_invoke.rs). libffi reads each argument from wherever the ABI put it.
• Return values: the JS function's result is converted to the declared return type. A BigInt keeps all 64 bits; numbers and booleans are converted. If the function throws, the call returns E_FAIL.
• Everything else: other delegates keep the shared Invoke.
• Engines: both the classic engine and the Node-API engine use it.

No API changes: asDelegate picks the right Invoke from the delegate's metadata.

Tests

cargo test -p runtime --lib --release:

• New tests: unit tests for signature classification, typed argument reading and result conversion, plus interop tests covering a JS function behind Int64 F(Single, Single, Single, Single) and a check that plain event-handler delegates keep the shared Invoke. All pass.
• Whole suite: 147 passed and 11 failed. The same 11 fail on main (the run_on_ui_thread, ui_dispatcher and a few XAML interop tests).

[coderabbitai]

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