feat(open-path): init never gates on the embedding model; open goes concurrent; slow opens narrate

A production restart storm measured 90,017ms for a single brain init vs
1,117ms quiet (~80x contention multiplier), traced to performInit() eagerly
awaiting the process-global WASM embedding engine before the VFS root even
existed. Every writer's open() queued on the one throttled model compile
(90-140s on throttled CPUs).

- VirtualFileSystem.doInitializeRoot() no longer embeds '/'. The root is
  system-tier plumbing nothing ever searches; when the default WASM engine
  is active it now gets an explicit all-zero placeholder vector
  (cosineDistance returns max distance for a zero vector, so it never ranks
  ahead of real content). deferEmbedding was considered and rejected: its
  landing path kicks the embed worker synchronously right after commit,
  which would still force the cold compile within milliseconds — just off
  the awaited path, not avoided. A registered native 'embeddings' provider
  (no cold-start cost, possibly a different dimension) still embeds the
  root for real, via the new Brainy.usesDefaultWasmEmbedder() seam.

- performInit()'s eager-embedding step now only STARTS the WASM engine warm
  in the background instead of awaiting it inline. embed()/embeddingManager
  already serialize concurrent callers on one shared init promise, so the
  first real embed() converges correctly either way; a failed warm narrates
  loudly instead of surfacing as a silent latency spike or an unhandled
  rejection. eagerEmbeddings: false still means no warm at all.

- FileSystemStorage.init() batches its ~8 independent bootstrap mkdirs
  (each creates its own full subtree via recursive:true, so none depend on
  the others existing) into one Promise.all. The restore-completion step
  and initializeCounts() stay strictly sequential — they have real order
  dependencies on rootDir and systemDir respectively.

- performInit() now times five phases (storage init / generation-store
  open+fold / index init+gate / VFS bootstrap / embedding-warm-started) and
  logs one warning with the per-phase breakdown when total open exceeds
  2000ms; silent otherwise.

src/vfs/VirtualFileSystem.ts

@@ -89,6 +89,16 @@ export class VirtualFileSystem implements IVirtualFileSystem {
   // Uses deterministic UUID format for storage compatibility
   private static readonly VFS_ROOT_ID = '00000000-0000-0000-0000-000000000000'
 
+  // OPEN-PATH FIX: the dimension of the placeholder vector given to the VFS
+  // root when it is first created (see `doInitializeRoot`). Mirrors the
+  // built-in WASM embedding engine's fixed, hardcoded output size
+  // (all-MiniLM-L6-v2 — see src/embeddings/candle-wasm/src/lib.rs
+  // HIDDEN_SIZE and src/embeddings/EmbeddingManager.ts). Deliberately NOT
+  // derived from `brain.dimensions` — this constant only applies to the
+  // default-embedder branch, where the true output dimension is this fixed
+  // value by construction, never a moving target.
+  private static readonly VFS_ROOT_VECTOR_DIMENSIONS = 384
+
   /**
    * Construct a VFS bound to a Brainy instance.
    *
@@ -260,6 +270,40 @@ export class VirtualFileSystem implements IVirtualFileSystem {
     try {
       console.log('VFS: Creating root directory (fixed ID: 00000000-0000-0000-0000-000000000000)')
 
+      // OPEN-PATH FIX: the VFS root is Brainy's own system-tier plumbing — it
+      // is hidden from find()/getNounCount()/stats() by default and nothing
+      // ever runs a semantic search against it — so it needs no REAL
+      // embedding. Historically this add() always called embed('/'), which
+      // meant every writer's FIRST-EVER open forced the process-global WASM
+      // engine to cold-compile its model (measured 90-140s on throttled
+      // CPUs) before the brain could even finish init(). This branch only
+      // runs once per store (the root already exists — with a real vector —
+      // in every previously-opened production store; reopening never re-adds
+      // it), so the fix applies only to brand-new stores.
+      //
+      // Chosen fix: an explicit all-zero vector, not `deferEmbedding: true`.
+      // `deferEmbedding` looked attractive (ack now, embed later) but its
+      // landing path (`kickEmbedWorker()`, called synchronously right after
+      // commit — see brainy.ts add()/update()) would still force the WASM
+      // engine to cold-compile within milliseconds of open, just off the
+      // awaited path instead of never paying it at all — worse than the
+      // explicit-vector path, which never touches the engine for this row.
+      // An all-zero vector is safe: `cosineDistance` (src/utils/distance.ts)
+      // explicitly returns the MAXIMUM distance whenever either operand's
+      // norm is zero, so the root can never rank ahead of real content in a
+      // similarity search, and HNSW indexes it like any other vector.
+      //
+      // Only the default WASM engine gets this treatment — its output
+      // dimension (384) is fixed and hardcoded, so the placeholder can never
+      // mis-pin `brain.dimensions` for it. A plugin-registered native
+      // 'embeddings' provider has no cold-compile cost AND may use a
+      // different dimension, so it keeps embedding the root for real (same
+      // as before this fix) rather than risk pinning the wrong dimension
+      // ahead of the caller's own first real embed.
+      const rootVector = this.brain.usesDefaultWasmEmbedder()
+        ? new Array(VirtualFileSystem.VFS_ROOT_VECTOR_DIMENSIONS).fill(0)
+        : undefined
+
       await this.brain.add({
         id: rootId,  // Fixed ID - storage ensures uniqueness
         data: '/',
@@ -271,7 +315,8 @@ export class VirtualFileSystem implements IVirtualFileSystem {
         // public AddParams.visibility union ('public' | 'internal') — this is the single
         // sanctioned internal setter, hence the cast.
         visibility: 'system' as 'public' | 'internal',
-        metadata: this.getRootMetadata()
+        metadata: this.getRootMetadata(),
+        ...(rootVector ? { vector: rootVector } : {})
       })
 
       return rootId