feat(igla): Wave Loop 899 — module-scope [617][2]^6 Pt non-power-of-two outer-dimension array-of-struct variable from call with indexed signed writes #1901

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

• Variant A: module-scope [617][2]^6 Pt variable initialized from a pure make_grid(0) call, with indexed signed field writes and assert_eq read-back inside a bench block.
• Expected: 39,488 elements, 1,263,616-bit packed vector, ~1.206 MiBit.
• Continues the post-1-MiBit boundary stress test.
• Zero compiler / reference-model / FROZEN_HASH changes expected.

Refs the Wave Loop autopilot plan.

User Scenarios

• Given a module-scope [617][2]^6 Pt array-of-struct variable initialized from a pure make_grid(0) call, when indexed signed field writes execute and assert_eq read-back runs inside a bench block, then the compiler emits a 39,488-element, 1,263,616-bit packed vector (~1.206 MiBit) with zero compiler/reference-model/FROZEN_HASH changes.

Requirements

FR-001: The compiler MUST support module-scope array-of-struct variables with non-power-of-two outer dimension [617][2]^6 Pt.
FR-002: The compiler MUST correctly initialize such variables from a pure make_grid(0) call.
FR-003: The compiler MUST handle indexed signed field writes to the array-of-struct elements.
FR-004: The compiler MUST verify read-back correctness via assert_eq inside a bench block.
FR-005: The packed vector MUST contain exactly 39,488 elements totaling 1,263,616 bits (~1.206 MiBit).
FR-006: The change MUST require zero modifications to the compiler, reference model, or FROZEN_HASH.

Success Criteria

• t27c specs/tri/collections/array.t27 compiles with exit 0
• t27c specs/tools/trinity/tri/bench.t27 compiles with exit 0
• t27c specs/memory/graph/scope.t27 compiles with exit 0
• Benchmark run produces 39,488 elements, 1,263,616-bit packed vector, ~1.206 MiBit
• 5 tests pass for Wave Loop 899 Variant A scenario

Boundary

• specs/tri/collections/array.t27
• specs/tools/trinity/tri/bench.t27
• specs/memory/graph/scope.t27