feat: scaled transact budgets + labeled timeout diagnostics + envelope docs

- The transact apply budget scales with the batch — max(30s, opCount x 2s) —
  or is exactly the caller's new TransactOptions.timeoutMs. A flat 30s cap
  silently limited honest bulk work to ~15 operations on network-attached
  storage (~2s/op measured in a production import) while looking generous for
  small batches. Internal batch paths (removeMany chunks) get the same
  scaling via the shared transactTimeoutBudget helper.
- TransactionTimeoutError now reports the operation it stopped at as i/N with
  the operation's name, elapsed vs budgeted time, and states the batch rolled
  back atomically and is retryable; context carries the fields
  programmatically.
- The transact operational envelope is documented (optimistic-concurrency
  guide): budget math, chunking with ifAbsent idempotency, when to reach for
  addMany vs transact, and the precompute pattern (embedBatch + per-op
  vector) that keeps inference out of the commit path.
- Embedding claims made honest per an end-to-end probe of the built dist:
  batch and single embedding paths produce bit-identical vectors and a
  vector-supplied add is fully searchable, but batch throughput on the
  default WASM engine measures comparable to sequential (~160ms/text) — the
  5-10x speedup claim in the addMany JSDoc is replaced with the measured
  reality and the actual win (inference outside the budgeted write path).

src/transaction/Transaction.ts

@@ -37,6 +37,24 @@ const DEFAULT_OPTIONS: Required<TransactionOptions> = {
   maxRollbackRetries: 3
 }
 
+/**
+ * The apply-phase budget for a batch of `opCount` operations.
+ *
+ * An explicit override wins untouched. Otherwise the budget SCALES with the
+ * batch: `max(30 000 ms, opCount × 2 000 ms)`. The per-op term is calibrated
+ * from field data — bulk imports on network-attached disks measure ~2 s per
+ * operation (each op pays canonical writes + fsync + index maintenance) — so
+ * a flat 30 s budget silently capped honest work at ~15 operations while
+ * looking generous for small batches. Scaling keeps small transacts
+ * fast-failing and gives bulk ones a budget proportional to the work they
+ * actually asked for; a trip still rolls back atomically and throws a
+ * retryable, fully-labeled TransactionTimeoutError.
+ */
+export function transactTimeoutBudget(opCount: number, override?: number): number {
+  if (override !== undefined) return override
+  return Math.max(30_000, opCount * 2_000)
+}
+
 /**
  * Transaction class
  */
@@ -114,7 +132,11 @@ export class Transaction implements TransactionContext {
         // into the catch below and rolls back like any other failure — it must
         // never bypass rollback.
         if (Date.now() - this.startTime > this.options.timeout) {
-          throw new TransactionTimeoutError(this.options.timeout, i)
+          throw new TransactionTimeoutError(this.options.timeout, i, {
+            elapsedMs: Date.now() - this.startTime,
+            totalOperations: this.operations.length,
+            operationName: this.operations[i]?.name
+          })
         }
 
         const operation = this.operations[i]

src/transaction/errors.ts

@@ -77,11 +77,24 @@ export class InvalidTransactionStateError extends TransactionError {
 export class TransactionTimeoutError extends TransactionError {
   constructor(
     timeoutMs: number,
-    operationIndex: number
+    operationIndex: number,
+    telemetry?: {
+      elapsedMs?: number
+      totalOperations?: number
+      operationName?: string
+    }
   ) {
+    const progress =
+      telemetry?.totalOperations !== undefined
+        ? `${operationIndex}/${telemetry.totalOperations}`
+        : String(operationIndex)
+    const name = telemetry?.operationName ? ` ('${telemetry.operationName}')` : ''
+    const elapsed =
+      telemetry?.elapsedMs !== undefined ? `${telemetry.elapsedMs}ms elapsed, ` : ''
     super(
-      `Transaction timed out after ${timeoutMs}ms at operation ${operationIndex}`,
-      { timeoutMs, operationIndex }
+      `Transaction timed out at operation ${progress}${name} — ${elapsed}budget ${timeoutMs}ms. ` +
+        `The batch rolled back atomically; retry with a higher timeoutMs or a smaller batch.`,
+      { timeoutMs, operationIndex, ...telemetry }
     )
     this.name = 'TransactionTimeoutError'
   }