feat(igla): Wave Loop 895 — module-scope [609][2]^6 Pt non-power-of-two outer-dimension array-of-struct variable from call with indexed signed writes #1853

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

• Variant A: module-scope [609][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: 38,976 elements, 1,247,232-bit packed vector, ~1.190 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 t27 spec defining a module-scope [609][2]^6 Pt array-of-struct variable initialized from make_grid(0), When the spec is compiled and the bench block executes with indexed signed field writes and assert_eq read-back, Then the bench passes with 38,976 elements packed into a 1,247,232-bit vector (~1.190 MiBit).

Requirements

FR-001: The spec MUST define a module-scope variable of type [609][2]^6 Pt initialized via make_grid(0).
FR-002: The bench block MUST perform indexed signed field writes to the array-of-struct elements.
FR-003: The bench block MUST verify correctness via assert_eq read-back of written values.
FR-004: The packed vector representation MUST contain exactly 38,976 elements totaling 1,247,232 bits (~1.190 MiBit).
FR-005: The spec MUST compile and bench execute without compiler, reference-model, or FROZEN_HASH changes.

Success Criteria

• t27c specs/tri/collections/array.t27 compiles and t27b bench specs/tri/collections/array.t27 runs the bench block with exit 0
• The bench output reports 38976 elements and 1247232 bits packed vector size
• File bootstrap/stage0/FROZEN_HASH remains unchanged

Boundary

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