fix(8.0): drive query-cap off MemAvailable + floor auto-detected caps

The auto-detected query-limit cap was computed from os.freemem() (MemFree),
which excludes reclaimable page cache. On hosts that memory-map large index
files the cache holding those pages dominates RAM, so MemFree collapses to a
sliver and the cap cratered to its floor on perfectly healthy machines,
rejecting legitimate find() calls.

- getAvailableMemory() now prefers /proc/meminfo MemAvailable (counts
  reclaimable cache), falling back to os.freemem() off-Linux, then a 2 GB
  constant where no OS module is available.
- Auto-detected caps (container + free-memory tiers) are floored at
  MIN_AUTO_QUERY_LIMIT (10_000); a transient low reading can never throttle
  queries to a near-useless ceiling. Consumer-supplied maxQueryLimit /
  reservedQueryMemory bypass the floor — an explicit caller knows their box.

Also scrubs two stale comments referencing the removed cloud storage
adapters and the retired mmap-vector backend: 8.0's native vector provider
persists its own .dkann file via getBinaryBlobPath, so the old rootDirectory
hook does not apply on this line.

Tests: memoryLimits 26/26 (4 new floor regressions), unit 1398/1398,
find-limits + db-mvcc + api-parameter-validation 37/37.

src/brainy.ts

@@ -770,10 +770,11 @@ export class Brainy<T = any> implements BrainyInterface<T> {
 
       // Wire the connections codec (2.4.0 #3). When the graph:compression
       // provider is registered AND the metadata index exposes a stable
-      // idMapper, inject a codec that encodes HNSW connections as
-      // delta-varint blobs at save time and decodes on load. The blob
-      // primitive itself works on every brainy 7.25.0 adapter, so unlike the
-      // mmap-vector backend this layer engages even on cloud adapters.
+      // idMapper, inject a codec that encodes JS HNSW connections as
+      // delta-varint blobs at save time and decodes on load. It rides the
+      // storage adapter's generic binary-blob primitive (both the filesystem
+      // and memory adapters implement it), so it is independent of any native
+      // vector provider's single-file on-disk format.
       this.wireConnectionsCodec()
 
       // 8.0 generational MVCC: if crash recovery rolled back an uncommitted
@@ -10533,9 +10534,11 @@ export class Brainy<T = any> implements BrainyInterface<T> {
    * metadata index exposes a stable idMapper. Failures are non-fatal: HNSW
    * keeps working via the legacy JSON-array path.
    *
-   * Unlike the mmap-vector backend, this layer does NOT require a real local
-   * path — the binary-blob primitive saves the compressed bytes through the
-   * storage adapter directly, so the codec is engaged on cloud adapters too.
+   * This layer does NOT require a real local path — the binary-blob primitive
+   * saves the compressed bytes through the storage adapter directly, so the
+   * codec works on the memory adapter as well as filesystem. (A native vector
+   * provider, by contrast, persists its own single `.dkann` file and has no
+   * per-node connection lists, so it is skipped — see the feature-detect below.)
    * Format convergence is lazy: pre-2.4.0 nodes load via the legacy path,
    * then the next dirty save writes the compressed form (and the legacy
    * field of saveVectorIndexData becomes empty), so all reads converge over time