Remove write() return value and simplify TopicWriter API

- Fix seqNo renumbering bug: messages written before session initialization are now properly renumbered after receiving lastSeqNo from server
- Remove return value from write() method (now returns void) to simplify API
- Remove resolveSeqNo() method and related seqNo shift tracking infrastructure
- Update tests to remove assertions on write() return values
- Add changeset describing bug fix and API simplification

Author: polRk

.changeset/heavy-facts-happen.md

@@ -0,0 +1,28 @@
+---
+'@ydbjs/topic': minor
+---
+
+Fix seqNo renumbering bug and simplify TopicWriter API.
+
+**Bug fix:**
+
+- Fixed issue where messages written before session initialization were not renumbered after receiving `lastSeqNo` from server. Previously, auto-generated seqNo started from 0 and were not updated when server provided actual `lastSeqNo`, causing seqNo conflicts. Now messages are properly renumbered to continue from server's `lastSeqNo + 1`.
+
+**API changes:**
+
+- `TopicWriter.write()` no longer returns sequence number (now returns `void`) to simplify API and prevent confusion about temporary vs final seqNo values
+
+**Migration guide:**
+
+- If you were storing seqNo from `write()` return value, use `flush()` instead to get final seqNo:
+
+  ```typescript
+  // Before
+  let seqNo = writer.write(data)
+
+  // After
+  writer.write(data)
+  let lastSeqNo = await writer.flush() // Get final seqNo
+  ```
+
+- User-provided seqNo (via `extra.seqNo`) remain final and unchanged - no migration needed for this case.

.changeset/remove-write-seqno-return.md

@@ -37,7 +37,6 @@ import { isRetryableError } from '@ydbjs/retry'
 import { assign, enqueueActions, sendTo, setup } from 'xstate'
 import { defaultCodecMap } from '../codec.js'
 import { WriterStream, type WriterStreamReceiveEvent } from './stream.js'
-import { SeqNoShiftBuilder } from './seqno-shift-builder.js'
 import type { TopicWriterOptions, WriterContext, WriterEmitted, WriterEvents, WriterInput } from './types.js'
 import { loggers } from '@ydbjs/debug'
 
@@ -268,8 +267,7 @@ let writerMachineFactory = setup({
 		 *
 		 * Mode-specific behaviour:
 		 * - Manual seqNo: compact the window, update bookkeeping, keep user-provided seqNo as-is
-		 * - Auto seqNo: compact the window, renumber remaining messages, emit `SeqNoShift` segments so
-		 *   `TopicWriter.resolveSeqNo()` can translate temporary numbers into the final ones
+		 * - Auto seqNo: compact the window, renumber remaining messages sequentially
 		 *
 		 * @param enqueue - XState enqueue helper for scheduling actions
 		 * @param event - init response with session metadata
@@ -375,23 +373,15 @@ let writerMachineFactory = setup({
 
 			let newBufferStart = firstPendingIndex
 
-			let shiftBuilder = new SeqNoShiftBuilder()
-
 			// Renumber the remaining messages sequentially so we continue where the server left off.
 			for (let i = firstPendingIndex; i < bufferEndIndex; i++) {
 				let message = context.messages[i]
 				if (!message) continue
 
-				let oldSeqNo = message.seqNo
-				let newSeqNo = nextSeqNo
+				message.seqNo = nextSeqNo
 				nextSeqNo++
-
-				shiftBuilder.addShift(oldSeqNo, newSeqNo)
-				message.seqNo = newSeqNo
 			}
 
-			let seqNoShifts = shiftBuilder.build()
-
 			let inflightSize = context.inflightSize - acknowledgedSize - pendingSize
 			let bufferSize = context.bufferSize + pendingSize
 			let garbageSize = context.garbageSize + acknowledgedSize
@@ -413,7 +403,6 @@ let writerMachineFactory = setup({
 				sessionId: event.data.sessionId,
 				lastSeqNo: lastSeqNo,
 				nextSeqNo,
-				...(seqNoShifts.length ? { seqNoShifts } : {}),
 			}))
 		}),