fix: reject @locals values that overflow word-alignment - #2838

Summary

• @locals(65535) (and 65533, 65534) causes a panic because num_locals.next_multiple_of(4) overflows u16
• Adds a bounds check in the parser grammar that rejects @locals values above 65532 with a descriptive error
• 65532 is the largest u16 value that can be safely word-aligned (rounded up to a multiple of 4)

Details

The overflow occurs in fmp_start_frame_sequence and fmp_end_frame_sequence (crates/assembly/src/fmp.rs) at:

let locals_frame = Felt::from_u16(num_locals.next_multiple_of(WORD_SIZE as u16));

For num_locals = 65535, next_multiple_of(4) = 65536, which exceeds u16::MAX. In debug builds this panics; in release builds it wraps to 0.

The fix validates at parse time using the existing InvalidLocalsAttr error variant, so the user gets a clear diagnostic:

error: invalid procedure @locals attribute
  --> test:2:5
  |
2 |     @locals(65535)
  |     ^^^^^^^^^^^^^^ number of locals exceeds the maximum of 65532

Test plan

• Added locals_overflow_rejected test: verifies @locals(65535) produces the expected error
• Added locals_max_valid_accepted test: verifies @locals(65532) is accepted
• All 86 existing miden-assembly-syntax tests pass

Fixes #2829