Diverse cleanups and docs in the JS and kvvfs pieces. Functional changes: (A) ensure...

Diverse cleanups and docs in the JS and kvvfs pieces. Functional changes: (A) ensure that the 'i32' JS/WASM func arg/result conversion works properly with a 64-bit-memory WASM build (which we neither use nor test but [https://webassembly.org/news/2025-09-17-wasm-3.0/ | the newly-ratified WASM 3.0] brings within potential reach). (B) Fix sqlite3_js_posix_create_file() to not deallocate its input array if the client passes in raw memory (the library has never used it that way but the API permits it).

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