Stack-allocate trivial value_object/value_array argument temporaries

[dimokol]

Addresses the argument-marshaling side of #27553.

When a value type is trivially constructible and destructible and does not require over-alignment (alignof(T) <= STACK_ALIGN), its argument temporaries no longer round-trip through new T() plus destructor bookkeeping. The registration passes sizeof(T) and a triviality flag; toWireType places the temporary on the wasm stack when the invoker brackets the call in stackSave/stackRestore (a null destructors argument is that contract), zero-filled so unregistered fields and padding match the heap path's value-initialization. The bracket restores the frame in a finally, so a throwing argument conversion or callee cannot leak stack. Over-aligned types keep the heap path, as do callers that defer destruction (emval returns, property setters), Asyncify builds, and JSPI-async invokers, which outlive the frame. Field/element writes skip their per-write destructors array when the element type registers no destructor, which turns out to be the dominant source of per-call garbage in large modules (V8 sinks those arrays in small benchmarks but not at scale). The AOT generator mirrors the type shape so invoker signatures stay in sync, and libsigs.js is regenerated for the new registration parameters.

Measured on box3d.js (a real embind physics binding) rebuilt with this branch, forced-GC heapUsed deltas, median over 9 rounds of 20k calls:

call	before	after
getter taking one 3-field value_object id	96 B/call	0
getter with id + value_array vec3 argument	208	16
raycast (id, two vec3, filter in; result object out)	179	37
world step (id + two scalars)	125	61

A stock rebuild with an unpatched toolchain reproduces the before column exactly. The remaining nonzeros are value-type returns, which still materialize fresh objects by design and are out of scope here.

other.test_embind, other.test_embind_aot_js, and other.test_embind_no_dynamic pass locally.

Draft because two things are still missing and I'd like direction before writing them: dedicated tests for the new behavior (a trivial POD value type exercising the stack path, plus an allocation regression check if that fits the suite), and the ChangeLog entry. Note the registration arity change means objects built against an older bind.h need a rebuild.