Preserve nullable matrix types

[luisleo526]

What changed

• preserve the exact matrix element type when Pine assigns bare na to a matrix destination
• carry matrix target typing through declarations, := reassignments, ternary/if/switch selections, and persistent first-reach storage
• preserve typed matrix arguments and UDT fields, including positional, keyword, and omitted arguments
• keep established matrix-returning builtins typed in functional and method form
• add focused lifecycle coverage for top-level and callable matrix state

Why

Pine permits matrix IDs to be na. Codegen previously lowered bare na as scalar na<double>(), even when the destination was a typed matrix. That lost the matrix element type and could also turn persistent selection initializers into /* unknown */ or silently drop them.

This change is the codegen half of the nullable-matrix lifecycle fix. It targets the nullable runtime state merged in pineforge-engine PR #127 and was revalidated against merged engine 7b765aa702cc.

Impact

Generated strategies now retain matrix identity and element typing across nullable lifecycle operations. Incompatible element-type reassignment remains rejected.

This PR deliberately does not add nullable arrays, matrix assignment alias semantics, matrix-returning UDF or method inference, general untyped var x = na inference, or broader UDT identity behavior.

Validation

• codegen full suite: 2,461 passed / 2 expected skips against merged engine 7b765aa702cc
• clean merged-engine rebuild and CTest: 115/115 passed
• exhaustive cross-repository N=3 matrix: 8/8, early stop disabled, publication gate passed
• engine corpus: 252/252 executed; 247 Excellent / 4 Strong / 1 documented anomaly
• forced-no-cache scraper gate: 416/416 accounted, zero compile/driver/unresolved timeout, 0 DOWN
• independent final review: no in-scope blocker
• production diff SHA-256: 5cab0779b4b4661ace209972656c17e9fd734588d0eb4b560b875dc36b15c33b