fix(bindgen): fix async future JS codegen producing invalid output

[wondenge]

The async future intrinsic templates produced invalid JavaScript that failed to parse when transpiling any component with future types. These bugs made P3 async futures completely unusable.

• Removed a stray closing parenthesis in the elementTypeRep range check
• Changed self to this in four places inside drop() - in JavaScript class methods, self refers to globalThis, not the instance
• Used the current variant name instead of always emitting futureWrite, which caused a duplicate declaration when rendering futureRead
• Same for futureCancelRead which was emitted for both cancel variants
• Renamed the second futureIdx parameter to futureEndIdx — the duplicate left futureEndIdx undefined at runtime
• Also fixed the matching duplicate in the debug log object
• Added the missing throw keyword before new Error in the instanceof guard, which was constructing an error and silently discarding it
• Removed a broken line that called .remaining(bufferID) without the BUFFER_MGR receiver and had no closing brace
• Added componentIdx to the cancel function parameter list and debug log, since the function body referenced it but never received it
• Added the missing lookup to fetch the future from the global map before comparing element type reps
• Replaced streamEndIdx with futureEndIdx, a copy-paste leftover from the stream equivalent
• Added the missing closing parenthesis in the instanceof check