fix(add): empty string is real data, not a missing field

validateAddParams() treated '' as falsy and rejected it with "Missing
required field 'data'" — so a legitimate empty file's first write always
failed. Only null/undefined data (with no vector either) is genuinely
absent; '' is real content. Fixed the check, plus the identical bug in
validateUpdateParams() (truncating a file to empty via overwrite hit the
same falsy check) and in update()/transact()'s update planner, where a
plain `Boolean(params.data)`/truthy check on the resolved vector would have
silently skipped both the deferred-embed marker and the eager re-embed for
an emptied value — a stale vector with no path to ever correct itself.

Verified end-to-end: vfs.writeFile('/empty.txt', '') now succeeds,
readFile() returns '', the file lists, and stat() reports size 0; the
existing "should reject empty string as data" tests (unit + integration)
asserted the old buggy behavior and are updated to assert the fixed
contract instead.

src/brainy.ts

@@ -3562,11 +3562,20 @@ export class Brainy<T = any> implements BrainyInterface<T> {
       // new `data`); otherwise new `data` re-embeds; otherwise the existing
       // vector is kept. Any vector change re-indexes HNSW below.
       let vector = existing.vector
+      // 'data' is a real new value whenever it's not null/undefined — an
+      // empty string ('') is legitimate content (e.g. truncating a file to
+      // empty via overwrite), matching validateUpdateParams's absent-vs-empty
+      // distinction. Using `Boolean(params.data)` here would treat '' as "no
+      // new data", silently skipping BOTH the deferred marker and the eager
+      // re-embed below — a stale vector left behind with no path to ever
+      // correct itself (a quiet loss, not the deferred-but-eventually-
+      // correct flicker the deferEmbedding contract promises).
+      const hasNewData = params.data !== undefined && params.data !== null
       // MT5 deferred re-embedding: the OLD vector keeps serving semantic
       // search — stale-but-present, never absent (the flicker law) — until
       // the background worker embeds the new data and swaps it atomically.
       const deferringEmbed =
-        params.deferEmbedding === true && Boolean(params.data) && !params.vector
+        params.deferEmbedding === true && hasNewData && !params.vector
       if (params.vector) {
         if (this.dimensions && params.vector.length !== this.dimensions) {
           throw new Error(
@@ -3574,13 +3583,13 @@ export class Brainy<T = any> implements BrainyInterface<T> {
           )
         }
         vector = params.vector
-      } else if (params.data && !deferringEmbed) {
+      } else if (hasNewData && !deferringEmbed) {
         vector = await this.embed(params.data)
       }
       // A deferred data change does NOT reindex now (the vector is unchanged;
       // the worker's atomic swap carries the real reindex later).
       const needsReindexing = Boolean(
-        (params.data && !deferringEmbed) || params.type || params.vector
+        (hasNewData && !deferringEmbed) || params.type || params.vector
       )
 
       // Always update the noun with new metadata
@@ -10501,6 +10510,11 @@ export class Brainy<T = any> implements BrainyInterface<T> {
     // Resolve the updated vector — mirror of update(): an explicit `vector`
     // always wins, new `data` re-embeds, otherwise the existing vector is
     // kept. Any vector change re-indexes HNSW below.
+    // 'data' is present whenever it's not null/undefined — '' is real
+    // content (see the identical hasNewData in update()); a plain truthy
+    // check would silently skip re-embedding an emptied value and leave a
+    // stale vector with no path to ever correct itself.
+    const hasNewData = params.data !== undefined && params.data !== null
     let vector = existing.vector
     if (params.vector) {
       if (this.dimensions && params.vector.length !== this.dimensions) {
@@ -10509,10 +10523,10 @@ export class Brainy<T = any> implements BrainyInterface<T> {
         )
       }
       vector = params.vector
-    } else if (params.data) {
+    } else if (hasNewData) {
       vector = await this.embed(params.data)
     }
-    const needsReindexing = Boolean(params.data || params.type || params.vector)
+    const needsReindexing = Boolean(hasNewData || params.type || params.vector)
 
     const newMetadata =
       params.merge !== false