Add wasm support - #204
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Closes #182.
The liblmdb.wasm previously shipped in runtimes/browser-wasm was unusable — mono-wasm can't load emscripten side modules (dotnet/runtime#75257) and nothing references that package path on browser. Removed.
Browser-wasm now works by shipping LMDB as compilable C source (wasm/ in the nupkg). A buildTransitive targets file adds it as a NativeFileReference when RuntimeIdentifier is browser-wasm; the consumer's wasm-tools workload compiles and statically links it, so there's no emscripten version coupling. Only prerequisite: dotnet workload install wasm-tools.
Required open flags (each verified load-bearing, see test app): WriteMap | NoLock | NoThreadLocalStorage | NoSync, with Flush(force: true) as the durability point. MEMFS mmaps aren't coherent with pwrite (default mode silently loses commits) and emscripten's msync only accepts mapping base addresses (sync-at-commit panics the env).
Encryption: AesGcmCipher can't exist on wasm (no platform AEAD) and managed page callbacks can't cross the wasm boundary, so the wasm build compiles in ChaCha20-Poly1305 (RFC 8439, same format as upstream crypto.c) + keyed BLAKE2b-256 via vendored Monocypher, installed natively — NativeChaCha20Poly1305Cipher / NativeBlake2bChecksum. Experimental; known limitation: encrypted envs ignore WriteMap (REMAP_CHUNKS), so same-session reads after commit see stale data — write → flush → reopen → read is the supported pattern.
Persistence: LightningBrowserStorage.MountAsync("/persist") mounts an IndexedDB-backed directory (IDBFS, linked automatically; opt out with WasmEnableIDBFS=false) and restores prior state; PersistAsync() checkpoints after Flush(true). Checkpoint durability — survives reload after each persist, not per commit. Single tab at a time.
Tests: src/LightningDB.WasmTest (kept out of the solution — needs wasm-tools) runs 15 in-browser scenarios including controls that assert the documented failure modes, plus cross-reload persistence phases. New wasm.yml workflow runs it headless on PRs; results are polled via document.title / DevTools endpoint because --virtual-time-budget stalls on IndexedDB.