BridgeJS: Support generic functions on imported JS APIs

[krodak]

Summary

Import half of generic function support (swiftwasm#398), split out of swiftwasm#787 per review. An imported @JSFunction can take a type parameter, so one declaration covers every bridged type:

@JSFunction func parse<T: BridgedSwiftGenericBridgeable>(_ json: String) throws(JSException) -> T

let user: User = try parse(jsonString)   // T inferred from the call site

T can be any primitive, String, JSValue, or a @JS struct/enum/final class, bare or as T?, [T], [String: T]. Also works on @JSClass inits, methods and statics, and for return-only generics. Exported generics are rejected with a diagnostic and land separately on top of this ABI.

How it works

• Values cross on the existing stack ABI. The only new thing on the wire is an i32 type ID per type parameter, so type-agnostic JS glue can pick the right lift/lower.
• Type IDs are pointer-based, not name-based (per BridgeJS: Support generic functions at the Swift and JavaScript boundary swiftwasm/JavaScriptKit#787 review): each type gets a BridgeJSTypeHandle, its address is the ID. Same-named types in different modules can't collide, and no existentials means it works under Embedded.
• Each module exports bjs_<Module>_register_type_handles, which hands its IDs to JS; JS pairs them with its codec table by index. Runs eagerly via a new afterInitialize instantiator hook, or lazily on first generic call (worker threads).
• Conformances are emitted for every @JS type, not just in modules that declare generics, module B can pass module A's type to a generic function (per BridgeJS: Support generic functions at the Swift and JavaScript boundary swiftwasm/JavaScriptKit#787 review).
• JS containers get one shared codec each (__bjs_arrayCodec etc.); the non-generic array/dict/optional paths now reuse them instead of inlining a copy per thunk (per BridgeJS: Support generic functions at the Swift and JavaScript boundary swiftwasm/JavaScriptKit#787 review on the stack ABI clones).

Builds without generics don't get the JS generic runtime, just a no-op registration hook per module with @JS types.

Test plan

• Codegen + link snapshots: free functions, inits, methods, statics, mixed/multiple params, return-only, wrapped forms
• Diagnostics: import constraints, export rejection
• ImportGenericAPITests: runtime round-trip of every bridgeable type through real JS, including JSON.parse
• Examples/Embedded builds a generic round-trip in CI
• Cross-module registration + runtime gating link tests