Pass and return FFI structs by value

[casselc]

Summary

• accept literal [:by-value [:struct ...]] types in foreign-fn and defcfn
• pass caller-owned pointed-to structs to C by value
• write aggregate C returns into an explicit caller-owned destination pointer and return that pointer
• support nested, large, multiple, blocking, and fixed-before-varargs aggregate calls
• reject null aggregate pointers before native entry and keep callbacks and exports fail-closed
• preserve lazy, lexically scoped native symbol resolution through the existing Scheme-adapter path

The explicit caller-buffer contract avoids hidden allocation and leaves ownership visible at the Jolt call site.

Stack

This is 2 of 3 and depends on #698. The reviewable commit for this layer is 918f6854 (the range after 62c6fd72).

GitHub native stacks do not support cross-fork branches, so this PR targets main and temporarily includes #698 in its cumulative diff. Once #698 lands, this PR narrows automatically to the by-value aggregate commit.

Validation

• rebased onto official v0.7.22 (8990efe2)
• pinned Chez 10.4.1 wrapper preflight
• focused FFI: bindings 19/19, exact widths 51/51, layouts 25/25, aggregate calls 16/16; public API tests passed
• layout and aggregate calls passed from source and a rebuilt standalone binary
• covers nested, large, multiple, blocking, fixed-before-varargs, and null-rejection cases
• Chez seed reminted to a two-pass fixpoint followed by a byte-stable one-pass remint
• Gambit seed regenerated and gambitseedcheck passed
• serial make -j1 test: 82 CI targets and 3 test targets passed
• unit gate: 1394/1394
• git diff --check
• fork CI passed on Linux x86-64 and macOS arm64