Add declarative FFI struct layouts

[casselc]

Summary

• add literal, data-only [:struct ...] descriptors through jolt.ffi/layout
• derive native size, alignment, and nested field offsets from lexically scoped Chez define-ftype declarations
• add layout-size, layout-alignment, field-offset, read-field, and write-field
• reject empty or malformed descriptors and unsupported field kinds at compile time
• compare flat, padded, and nested layouts against C ABI witnesses

Descriptors are immutable data rather than entries in a mutable registry. This initial surface supports fixed-size scalar fields and nested structs; arrays, unions, bitfields, packing, and recursive descriptors remain unsupported.

Stack

This is 1 of 3 in the FFI stack:

1. This PR: declarative struct layouts
2. Pass and return FFI structs by value #699: by-value aggregate calls
3. FFI: add scoped allocation helpers #700: scoped allocation helpers

GitHub native stacks do not support cross-fork branches, so the three PRs target main and are linked explicitly.

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; public API test passed
• layout public API passed from source and a rebuilt standalone binary
• Chez seed reminted to a two-pass byte fixpoint
• 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

[yogthos]

Nice, this should make FFI more ergonomic. :)