fix: transaction timeouts are a typed no-hot-retry contract; engine-side non-retry pinned; dead transaction path removed

A production incident: a native-provider op ground 38-40s inside a transaction,
blew the apply-phase budget, rolled back, and a downstream pipeline hot-retried
the identical operation into a 6-minute CPU storm. Brainy itself never
auto-retried the timeout; the gap was that TransactionTimeoutError only said
"retryable" in prose, with nothing machine-readable for a caller to branch on.

- TransactionTimeoutError gains two typed, always-true fields: retryable
  (a later attempt may succeed once the slowness resolves or the budget is
  raised) and hotRetryUnsafe (an immediate identical retry re-pays the full
  cost that just timed out and can cascade into a CPU storm -- callers must
  latch and back off, never loop). context's existing telemetry fields
  (timeoutMs, operationIndex, elapsedMs, totalOperations, operationName) are
  now documented as the caller's backoff inputs.
- Updated the "retryable" doc-prose sites (transact()'s timeoutMs option,
  transactionBudgetFloorMs, Transaction.execute()'s contract) to point at
  the new fields instead of bare prose.
- Regression pin (tests/unit/transaction/timeout-never-internally-retried.test.ts):
  an execution counter proves the engine never re-drives a timed-out
  operation, through both the single-op engine TransactionManager/Transaction
  drives for every single-record write, and add()'s upsert-race retry loop
  (which must exit on the first TransactionTimeoutError, never treat it like
  the lost-insert-race signal it retries on).
- Removed TransactionManager.executeTransactionWithResult -- zero callers
  anywhere in the codebase.

src/transaction/TransactionManager.ts

@@ -19,7 +19,6 @@
 import { Transaction } from './Transaction.js'
 import {
   TransactionFunction,
-  TransactionResult,
   TransactionOptions
 } from './types.js'
 import { TransactionError } from './errors.js'
@@ -105,34 +104,6 @@ export class TransactionManager {
     }
   }
 
-  /**
-   * Execute a transaction and return detailed result
-   */
-  async executeTransactionWithResult<T>(
-    fn: TransactionFunction<T>,
-    options?: TransactionOptions
-  ): Promise<TransactionResult<T>> {
-    const startTime = Date.now()
-    const transaction = new Transaction(options)
-
-    try {
-      const value = await fn(transaction)
-      await transaction.execute()
-
-      const executionTimeMs = Date.now() - startTime
-
-      return {
-        value,
-        operationCount: transaction.getOperationCount(),
-        executionTimeMs
-      }
-
-    } catch (error) {
-      // Transaction failed
-      throw error
-    }
-  }
-
   /**
    * Get transaction statistics
    */