Update uWebSockets submodule: replace maxRemainingBodyLength() with onDataV2

[Copilot]

Upstream uWebSockets removed maxRemainingBodyLength() from HttpResponse and replaced it with onDataV2, which passes maxRemainingBodyLength directly as a uint64_t callback parameter (0 = last chunk, UINT64_MAX = chunked transfer encoding in-progress).

Changes

• Submodule: bump uWebSockets to e42d69f ("Remove maxRemainingBodyLength(), add onDataV2")
• res_onStream: switch from onData(fn(data, bool last)) + res->maxRemainingBodyLength() call inside handler to onDataV2(fn(data, uint64_t maxRemainingBodyLength)); drops res capture from lambda
• res_onFullData: switch to onDataV2; replace last bool with maxRemainingBodyLength == 0; update preallocation hint from res->maxRemainingBodyLength() to data.size() + maxRemainingBodyLength (guards correctly against UINT64_MAX for chunked encoding)
• Remove: res_maxRemainingBodyLength wrapper function and its prototype registration
• Types: remove maxRemainingBodyLength(): bigint from HttpResponse in docs/index.d.ts

// Before
res->onData([res, sharedP, isolate](std::string_view data, bool last) {
    Local<Value> argv[] = {dataArrayBuffer, BigInt::NewFromUnsigned(isolate, res->maxRemainingBodyLength())};
});

// After
res->onDataV2([sharedP, isolate](std::string_view data, uint64_t maxRemainingBodyLength) {
    Local<Value> argv[] = {dataArrayBuffer, BigInt::NewFromUnsigned(isolate, maxRemainingBodyLength)};
});

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