Merge finalized Web transcript parent

# Conflicts:
#	.agents/notes/implemented/architecture/2026-07-30-session-end-seed-log-boundary.i18n.yaml

packages/core/session/src/index.ts

@@ -414,7 +414,9 @@ export class Session {
    * start here. Distinct from `header.seedLength`, the DURABLE fork-lineage
    * boundary: a resumed session's constructor seed is its full stored log,
    * while its header keeps the original fork value — this field is the
-   * in-process construction fact.
+   * in-process construction fact. An explicitly supplied empty seed has the
+   * same value as no seed (0); its `session/end-seed` event preserves the
+   * lifecycle distinction.
    *
    * Not persisted itself: a seeded session projects it into the log as the
    * `session/end-seed` event, which is what a consumer reading STORED history
@@ -430,7 +432,7 @@ export class Session {
   readonly firstLiveSeq: number
 
   constructor(id: SessionId, seed?: readonly SessionEvent[], header?: SessionHeader) {
-    if (seed) {
+    if (seed !== undefined) {
       // Validate the seed to the SAME invariants `append` enforces, so a
       // replay/fork (`ctx.sessions.create(id, { seed })`) cannot construct a
       // live log that no persistence backend could store: each event's `data`
@@ -467,7 +469,7 @@ export class Session {
     // captures the creation seed: no load-time write. Re-marking is skipped
     // because a cold session is resumed on first touch, so repeatedly opening
     // one must not grow its log per open.
-    if (this.firstLiveSeq > 0 && this.log.at(-1)?.type !== 'session/end-seed') {
+    if (seed !== undefined && this.log.at(-1)?.type !== 'session/end-seed') {
       this.append('session/end-seed', {})
     }
   }

packages/core/session/src/types.ts

@@ -256,7 +256,9 @@ export interface SessionEventMap {
   /**
    * Marks the end of a constructor seed. Events before it have smaller seq
    * values and came from the seed (resume, fork, or replay); this lifecycle
-   * produced none of them. This log-only event is the durable projection of
+   * produced none of them. An explicitly supplied empty seed puts the marker
+   * at seq 0, distinguishing an empty resumed session from a fresh session.
+   * This log-only event is the durable projection of
    * {@link Session.firstLiveSeq}. Its payload is empty — position and `time`
    * carry the meaning.
    *

packages/core/session/tests/fork.spec.ts

@@ -67,7 +67,7 @@ describe('SessionStore.fork', () => {
 
     const child = sessions.fork(source, undefined, SessionId('empty-child'))
 
-    expect(child.events).toEqual([])
+    expect(inherited(child)).toEqual([])
     expect(child.header).toMatchObject({
       id: SessionId('empty-child'),
       cwd: '/workspace',

packages/core/session/tests/properties.spec.ts

@@ -112,9 +112,9 @@ describe('Session properties', () => {
       const original = build(events)
       const replayed = new Session(SessionId(`replay-${counter++}`), [...original.events])
       expect(replayed.deriveMessages()).toEqual(original.deriveMessages())
-      // A non-empty replay grows by exactly one log-only boundary.
+      // Every explicit replay grows by exactly one log-only boundary.
       expect(replayed.events.slice(0, original.seq)).toEqual(original.events)
-      expect(replayed.seq).toBe(original.seq === 0 ? 0 : original.seq + 1)
+      expect(replayed.seq).toBe(original.seq + 1)
     }))
   })
 

packages/core/session/tests/session.spec.ts

@@ -194,6 +194,21 @@ describe('Session', () => {
     expect(replayed.firstLiveSeq).toBe(original.seq)
   })
 
+  it('marks an explicitly empty seed without marking a fresh session', () => {
+    const fresh = new Session(SessionId('fresh-empty'))
+    expect(fresh.events).toEqual([])
+
+    const resumed = new Session(SessionId('resumed-empty'), [])
+    expect(resumed.firstLiveSeq).toBe(0)
+    expect(resumed.events).toMatchObject([
+      { type: 'session/end-seed', seq: 0, data: {} },
+    ])
+
+    const reopened = new Session(SessionId('reopened-empty'), resumed.events)
+    expect(reopened.firstLiveSeq).toBe(1)
+    expect(reopened.events).toEqual(resumed.events)
+  })
+
   it('rejects pre-provider request headers and assistant messages on seed/load', () => {
     const requestHeader = {
       type: 'request/header', seq: 0, time: 1,