feat: session/projection push frame; tail block reads the watermark snapshot

packages/host/apiproxy/src/api/events.schema.ts

@@ -37,6 +37,9 @@ export const muxFrameSchema = z.discriminatedUnion('type', [
   z.object({ type: z.literal('question/resolved'), sessionId: sessionIdSchema, questionRpcId: rpcIdSchema, outcome: z.union([z.literal('answered'), z.literal('cancelled')]) }),
   // content/source reuse the wide passthroughs (both are merge-extensible in core).
   z.object({ type: z.literal('session/queued'), sessionId: sessionIdSchema, content: z.array(contentBlockSchema), source: z.looseObject({ kind: z.string() }), steering: z.boolean() }),
+  // value stays wide: it already passed its unit's own schema on the host,
+  // and deep-validating here would import every domain's schema into the carrier.
+  z.object({ type: z.literal('session/projection'), sessionId: sessionIdSchema, key: z.string().min(1), value: z.unknown(), seq: z.number().int().nonnegative() }),
   z.object({ type: z.literal('stream/error'), error: rpcErrorSchema }),
 ]) as unknown as z.ZodType<MuxFrame>
 

packages/host/apiproxy/src/api/events.ts

@@ -75,6 +75,15 @@ export type MuxFrame =
    * reconciliation key).
    */
   | { type: 'session/queued'; sessionId: SessionId; content: ContentBlock[]; source: MessageSource; steering: boolean }
+  /**
+   * One projection unit's finished value changed (session-projection RFC).
+   * Live push state, never logged — replay recomputes on the host (the
+   * tool-view posture). `value` is the unit's schema-validated view output;
+   * `seq` is the unit's watermark at emission. Clients keep one generic
+   * per-session value store under higher-seq-wins, seeded by the history
+   * tail page's projections block.
+   */
+  | { type: 'session/projection'; sessionId: SessionId; key: string; value: unknown; seq: number }
   | { type: 'stream/error'; error: RpcError }
 
 /**

packages/host/apiproxy/src/api/sessions.schema.ts

@@ -105,7 +105,8 @@ export const todoItemSchema = z.object({
  * deep-validating here would import every domain's schema into the carrier.
  */
 export const sessionProjectionsBlockSchema = z.object({
-  asOfSeq: z.number().int().nonnegative(),
+  // -1 = empty log (the lastSeq convention of session/subscribed).
+  asOfSeq: z.number().int().min(-1),
   values: z.record(z.string(), z.unknown()),
 }) as unknown as z.ZodType<SessionProjectionsBlock>
 

packages/host/apiproxy/src/api/sessions.ts

@@ -37,13 +37,16 @@ export interface HistoryEntry {
 
 /**
  * The projection baseline riding the history tail page: one synchronous cut
- * over every registered projection provider. `asOfSeq` equals the window tail
- * seq (the session's next-event seq at slice time) because the handler reads
- * it and every value with no await in between. A key absent from `values`
- * means the capability is absent (its domain plugin is unmounted).
+ * over every registered projection unit, read from the registry's watermark
+ * cache. `asOfSeq` is the seq of the last committed event every value
+ * reflects — the window tail event seq (`-1` for an empty log, mirroring
+ * `session/subscribed.lastSeq`), directly comparable with
+ * `session/projection` frame seqs under the client's higher-seq-wins rule. A
+ * key absent from `values` means the capability is absent (its domain plugin
+ * is unmounted).
  */
 export interface SessionProjectionsBlock {
-  /** The session seq the values are consistent with (window tail seq). */
+  /** Seq of the last event the values reflect; -1 for an empty log. */
   asOfSeq: number
   /** Whole current value per registered projection key. */
   values: Partial<SessionProjectionMap>