Wave Loop 889 — module-scope [597][2]^6 Pt non-power-of-two outer-dimension array-of-struct variable from call with indexed signed writes

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Next mechanical rung of the packed-vector array-of-struct ladder.

• Shape: module-scope [597][2]^6 Pt variable.
• Outer dimension 597 is non-power-of-two.
• Initialized from a pure helper call (e.g. make_grid(0)).
• Mutated via indexed signed field writes and read back with assert_eq inside a bench block.
• Expected packed vector width: 597 × 64 × 32 = 1,222,656 bits (~1.166 MiBit).

Acceptance criteria:

• t27c parse, icarus-lowerable, icarus-simulate, icarus-cocotb, seal --save all pass.
• Integration test added to bootstrap/tests/icarus_lowerable.rs.
• Zero changes to compiler, reference model, or FROZEN_HASH.
• PR opened with auto-merge.