feat(igla): Wave Loop 881 — module-scope [581][2]^6 Pt non-power-of-two outer-dimension array-of-struct variable from call with indexed signed writes

[gHashTag]

Wave Loop 881 mechanical witness.

• OUTER = 581, MID_IDX = 290
• Elements = 581 × 2⁶ = 37,184
• Packed vector = 37,184 × 32 = 1,189,888 bits ≈ 1.135 MiBit
• Continues the non-power-of-two outer-dimension array-of-struct packed-vector ladder past 1.1 MiBit.
• Zero compiler / reference-model / FROZEN_HASH changes for witness.
• Generator copy-hazard prevented by pre-run grep of all three stale-reference locations.

Validation matrix:

• t27c parse → PASS
• t27c icarus-lowerable → lowerable
• t27c icarus-simulate → PASSED (17 cycles)
• t27c icarus-cocotb → reference-model OK
• t27c seal --save → seal saved
• cargo test --release --test icarus_lowerable accepts_w881_bench_module_581x2p6_aos_var_call_write → passed
• Full suite → 341/0

Closes #1722