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

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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

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Closing — nothing left to land once this is measured against current master

Checked with the GitHub API against master at 10e3d5e71a970351d5d1c63f0dd88b77210dbffc. This PR touches exactly two paths:

Path	Status on master
bootstrap/tests/icarus_lowerable.rs	master already carries accepts_w881_bench_module_581x2p6_aos_var_call_write among its 352 accepts_w* tests; this hunk adds nothing new. The +1872 count is an artifact of an ~848-commit-old merge base, not new content. No test is removed by this close
.trinity/seals/scratch_w881_bench_module_581x2p6_aos_var_call_write.json	gitignored (.gitignore:120); master carries zero scratch_* seals, so re-adding this one would red the seal gate

So the entire remaining delta is one phantom seal. scripts/gen_w881.py is already on master too.

Worth recording rather than losing: this PR never contained a W881 narrative record — no docs/reports/FPGA_LOOP_CLOSEOUT_W881_*.md, no plan file, no .trinity/experience.md entry. master's experience log runs W898 → W884 and then jumps to W777, so W881–W883 have no entry there. Closing this PR discards nothing that ever existed; the gap is separate, and only the W883 half of it is recoverable (from #1815).

The branch wave-loop-881 is left in place.

Series close-out: #1810, #1813, #1842, #1844, #1847, #1850, #1852, #1854, #1856 and #1858 are all closed for this reason. #1815 was the only PR in the series with unique durable content and was rebuilt on top of master.