Decompose 590-line cast() into named pipeline stages

[ASDAlexander77]

Summary

Implements the first §5 item (giant-method decomposition) from docs/MLIRGen-refactoring-review.md.

cast() (590 lines) is a sequential pipeline, not a pure type dispatch — several blocks intentionally fall through when unhandled, and the ConstType case rewrites the target type mid-flow — so a TypeSwitch table would change semantics. Instead it is split into stage helpers returning std::optional<ValueOrLogicalResult> (nullopt = "not handled, continue the pipeline") plus LogicalResult verifiers, with a ~100-line driver that preserves the original stage order exactly:

verifyCastPreconditions (literal + strict-null) → enum unwrap → castViaToPrimitive → castToStringSpecialCases → castToInterfaceSpecialCases → castTupleLikeVariants → castToOptionalType → castToTaggedUnionType → const-dest rewrite (kept inline: mutates type) → castFromSourceSpecialCases (union/optional/any/opaque sources) → verifyFunctionCastRules → castExtensionFunctionType → verifyCastCompatibility → default CastOp.

Stage bodies are moved verbatim; the EXIT_IF_FAILED*/CAST_A macros compose correctly with the std::optional return (a failed result wraps and the driver returns it, identical to before).

Also includes the user's launch.json DEFAULT_LIB_PATH fix for the debug build, and repairs stray file corruption at the (this->*valueFactory) call site (five lines from #203 had been clobbered by a whitespace stub, most likely a stale editor buffer).

Test plan

• tslang rebuilt cleanly (vs2026 debug)
• New cast-focused smoke test — optional cast, tagged-union assignment, tuple, class→string via toString, object literal→interface, any unboxing — JIT output 42 7 1 a greeter x 5 as expected
• Prior smoke tests for Refactor theModule handling in MLIRGenImpl #201–Make GenContext cleanup pointers non-owning #204 paths unchanged (closures/inference, utility types, spread, generics, statics)
• MLIRGenTests.exe: 12/12 pass

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