1.8.3

v1.8.3: revert auto->WebGPU (memory crash) + add Indexing speed setting

The 70GB memory crash returned: 1.7.0 made "auto" prefer WebGPU again, and onnxruntime
-web's WebGPU backend still accumulates GPU/unified memory across reindexes (and plugin
reloads) until Obsidian dies — per-pass tensor disposal wasn't enough. Revert: "auto"
(and "wasm") resolve to multi-threaded WASM, which is memory-stable (verified flat at
~9GB across 3 back-to-back reindexes, no growth). WebGPU stays available only as an
EXPLICIT pin, with a clear warning in the setting + README.

Multi-threaded WASM is fast (~28s) but its shared heap holds several GB while loaded,
and that cost is roughly the same at 4 or 8 threads (it's the model's working set, not
per-thread) — only single-threaded is light (~2GB) but slow (~2.5min). Since there's no
fast-AND-light option, expose the trade-off: a new "Indexing speed" setting
(Light = 1 thread / Balanced = ~cores/3 / Fast = cores-2 capped 8), default Balanced,
wired via setEmbedThreads(); changing it recreates the engine (configureEnv re-applies
numThreads) and rebuilds, exactly like a device change. Incremental per-note updates
stay fast at any setting. No INDEX_VERSION change.