NFC: BridgeJS: Centralize type info on BridgeType, remove dead cleanup infrastructure

Summary

As recently discussed, this PR centralizes scattered type-specific knowledge as computed properties on BridgeType. Codegen reads these properties directly instead of re-deriving the same facts in each switch. Codegen still switches on BridgeType cases, grouping them when they share logic.

Also removes the cleanup infrastructure that became dead code after #635 and #636.

64 files changed, +3667/-5050, net -1383 lines.

What changed

BridgeType computed properties (BridgeJSSkeleton.swift)

Previously, type-specific knowledge (ABI shape, lowering method, optional convention, etc.) was scattered across switches in ExportSwift, ImportTS, BridgeJSLink, and JSGlueGen. Each file re-derived the same facts independently. This PR gathers that information into computed properties on BridgeType itself, so when adding or modifying a type, there's one place to look.

10 focused properties organized by consumer:

• ABI Shape - wasmParams, wasmReturnType, optionalConvention
• ExportSwift - accessorTransform, lowerMethod, usesStackLifting
• ImportTS - importParams, importReturnType
• JSGlueGen - nilSentinel, optionalScalarKind

Supporting types: OptionalConvention, NilSentinel, OptionalScalarKind, AccessorTransform, LowerMethod.

NilSentinel captures whether a type has a bit pattern that represents nil without an extra isSome flag:

• jsObject, swiftProtocol - sentinel 0 (object IDs start at 2)
• swiftHeapObject - null pointer
• caseEnum, associatedValueEnum - sentinel -1 (never a valid case index)

JSGlueGen helpers (JSGlueGen.swift)

Private helpers that switch on BridgeType for JS-side coercion: isSingleParamScalar(), stackLowerCoerce(), liftCoerce(), lowerCoerce(), varHint(). These consolidate the type-to-JS-coercion mapping that was previously spread across the four direction functions.

Dead cleanup infrastructure removal

After #635 and #636, no type produces cleanup code. Removed all of it:

• cleanupCode printer from IntrinsicJSFragment.PrintCodeContext
• tmpStructCleanups array and swift_js_struct_cleanup intrinsic handler
• _swift_js_struct_cleanup extern and wrapper from BridgeJSIntrinsics.swift
• Struct helper lower() no longer returns { cleanup } - just pushes fields
• Enum helper lower() returns caseId directly instead of { caseId, cleanup }
• init(unsafelyCopying:) no longer calls struct cleanup
• All optional/array/dictionary wrappers simplified

~1680 lines removed across source and snapshots.

Compositional optional handling

optionalLowerParameter and optionalLiftParameter compose T's existing fragment inside an isSome conditional instead of switching per type. New types added to lowerParameter or liftParameter get optional handling for free. Same for struct fields via structFieldLowerFragment.

Other generated code improvements

• JSValue optional lowering - __bjs_jsValueLower no longer called on null values; the isSome guard runs before lowering
• Struct/enum helper factory collapse - () => { return () => ({...}); } simplified to () => ({...}); call-site ()() becomes ()
• Typed defaults in optional lowering - 0.0 for floats, flag ? 1 : 0 for bools (previously 0 for everything)
• Dead code removal - unused functions in ExportSwift, JSGlueGen, BridgeJSSkeleton