Close out review-doc section 5; add const_cast<GenContext&> removal plan

[ASDAlexander77]

Summary

Closes §5 (giant-method decomposition) of docs/MLIRGen-refactoring-review.md and tees up §3.

• Code: the duplicated 5-parameter cast-callback lambda in mlirGenPropertyAccessExpressionBaseLogic is hoisted to a single castFn local shared by the ConstArrayType/ArrayType cases — the last real duplication in that function.
• Doc — §5 closed: investigating the remaining table entries showed the original measurements were stale: mlirGenClassVirtualTableDefinition was already reduced to ~96 lines by Make class vtable slot resolution O(1), deterministic, and kind-safe #200, and detectAccessLevel was always ~30 lines (the old survey mislabeled multi-line signatures). A fresh survey confirms nothing over 250 lines remains after Decompose 590-line cast() into named pipeline stages #205–Decompose 304-line inferType into per-template-kind helpers #207; further splitting is diminishing returns.
• Doc — A6 added: a phased plan for removing the 31 const_cast<GenContext &> mutations (§3), with a census by mutated field (9× funcOp, 4× typeParamsWithArgs, 3 loop-flag sites, 3× generatedStatements, 2× thisType, 2× stop(), 1× inferTypes, 2 in MLIRCodeLogic.h) and four independently shippable phases: legalize stop() via mutable; copy-on-override for downstream-only overrides (flagging unrestored mutations as the latent bugs §3 predicted); honest GenContext&/out-params for the inference family; parameterize the MLIRCodeLogic.h pokes.

Test plan

• tslang rebuilt cleanly (vs2026 debug)
• All seven JIT smoke tests (statics/gctors, generics, utility types, closures/inference, casts, object literals, type inference) produce identical output
• MLIRGenTests.exe: 12/12 pass

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