gen-c: a fixed-size [N]T parameter loses its size and the caller is never checked #3434

What

gen-c lowers a fixed-size array parameter to a bare pointer:

fn fixed(a: [4]u8) -> u8 { return a[0]; }

uint8_t fixed(uint8_t* a);

The length is gone. A caller passing a two-element array compiles silently.

The other two backends keep it: gen-rust emits [u8; 4], gen-verilog/Zig
keep [4]u8. C is the one column that drops it -- the same cross-backend
divergence pattern as #3403 and #3423.

Measured, not assumed

The obvious spelling does not fix it. Apple clang 21, -Wall -Wextra,
passing a 2-element array where 4 are required:

A plain [N] parameter decays to a pointer and checks nothing. Only
[static N] carries the contract.

Population

Counted by the emitter itself, over all 651 specs (583 of which emit C):

• 51 parameters in 24 functions across 12 files.

Concentrated in specs/ternary/bitnet_* (28), specs/vsa/ops (9) and
specs/compiler/lexer (3).

Scope

This is a guard for future and out-of-repo callers -- compiling the generated
corpus today produces 0 -Warray-bounds, so it finds no existing defect.
Its value is that the next short call is rejected instead of accepted.

Constraints found while scoping

• param_type_to_c also spells struct fields, where [static N] is a
  syntax error -- the fix has to be narrowed to parameter position, the same
  narrowing the *T -> &mut T lowering needed in fix(rust-backend): prefix the dereference, and give an out-pointer a safe type #3423.
• [static 0] is not valid C, so a [0]T parameter must keep the pointer.
• A const-named size is only safe if the file #defines it earlier.