feat(telemetry): adopt from the construction boundary — constructor seeds never re-export

A cursor-less adoption (process restart + resume, fork, seam-module
reload) replayed the session's full log from seq 0, re-exporting
history that already left the process — a resume re-billed its entire
stored log on every restart, and a fork re-shipped the parent's prefix
under the child's id, doubling query-time counts on OTLP backends with
no native ingest dedupe.

dsh-session now exposes the fact the constructor already validated but
discarded: Session.firstLiveSeq, the constructor-seed length — the
first seq appended in this process. header.seedLength cannot serve
here: it is the durable fork-lineage boundary, and a resumed session's
constructor seed is its full stored log while the header keeps the
original fork value (llm-replay and session-query-sqlite depend on
that meaning). Constructor seeds also never publish on the
session/event firehose, so adoption replaying them was inconsistent
with the system's own publication semantics.

Adoption's cursor-less fallback starts at firstLiveSeq; seed events
still feed the chunk projection, so mid-step continuations re-drop
after a resume. Fork streams are no longer self-contained: records now
carry session.seed_length (with the existing session.parent_id) so
receivers stitch the child's stream onto the parent's. Accepted cost,
consistent with at-most-once delivery and recorded in the revival
Agent Note: a resume no longer backfills records a previous process
failed to deliver — a deployment with that requirement needs the
deferred outbox, not replay.

Pinned red-first: seeded adoption exports nothing (assertion reversed
from the prior seed-readback test, obsolete behavior changed with its
test), resume-shaped seed rebuilds the projection without exporting,
and fork records carry the stitch attributes.

packages/core/session/src/index.ts

@@ -300,6 +300,19 @@ export class Session {
     return this.header.id
   }
 
+  /**
+   * The first seq appended IN THIS PROCESS: the length of the constructor
+   * seed (0 without one). Events below it entered through construction —
+   * replay, fork, or resume — and were never published on the `session/event`
+   * firehose (constructor seeds do not emit), so consumers that replay the
+   * log as a publication substitute (telemetry adoption) 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
+   * and is deliberately not persisted.
+   */
+  readonly firstLiveSeq: number
+
   constructor(id: SessionId, seed?: readonly SessionEvent[], header?: SessionHeader) {
     if (seed) {
       // Validate the seed to the SAME invariants `append` enforces, so a
@@ -332,6 +345,7 @@ export class Session {
         this.log.push(deepFreeze(snapshot))
       }
     }
+    this.firstLiveSeq = this.log.length
     this.header = snapshotSessionHeader(id, header)
   }