feat(domain,infra,app): borne summary_md du handoff + durcissement seam LLM (LS5)

Clôt le must-have perf du handoff :
- domain : HANDOFF_SUMMARY_MAX_CHARS + bound_handoff_summary (helpers privés), re-export lib.
- infrastructure : TURN_LINE_MAX_CHARS + élision dans render_turn (summarizer), re-exports.
- application : borne entre fold et save (conversation/record), borne défensive
  dans resolve_handoff (agent/lifecycle) + module test.
- seam LLM prêt mais NON activé (aucun LLM câblé).
- tests (QA, verts) : conversation_log, conversation_record, agent_lifecycle.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/application/src/conversation/record.rs

@@ -2,7 +2,10 @@
 
 use std::sync::Arc;
 
-use domain::{ConversationId, ConversationLog, ConversationTurn, HandoffStore, HandoffSummarizer};
+use domain::{
+    bound_handoff_summary, ConversationId, ConversationLog, ConversationTurn, HandoffStore,
+    HandoffSummarizer, HANDOFF_SUMMARY_MAX_CHARS,
+};
 
 use crate::error::AppError;
 
@@ -63,6 +66,9 @@ impl RecordTurn {
     /// 1. **append** the turn to the canonical log (source of truth) ;
     /// 2. **load** the previous handoff (`None` on a first checkpoint) ;
     /// 3. **fold** `prev` with the single new turn (incremental, best-effort) ;
+    /// 3b. **bound** the folded handoff's summary to [`HANDOFF_SUMMARY_MAX_CHARS`] — a
+    ///     **universal cap independent of the summarizer** (heuristic *or* a future LLM):
+    ///     idempotent, never rejecting, so it never blocks the checkpoint (LS5) ;
     /// 4. **save** the advanced handoff.
     ///
     /// The append happens **first** so the durable source of truth is never behind
@@ -92,6 +98,13 @@ impl RecordTurn {
             .fold(prev, std::slice::from_ref(&turn))
             .await;
 
+        // 3b. **Universal bound** (LS5): clamp the resume summary to
+        //     `HANDOFF_SUMMARY_MAX_CHARS`, *independently of the summarizer*. Whatever
+        //     produced the handoff — the heuristic or a future LLM summarizer — the
+        //     persisted (and later injected) summary is size-bounded. Idempotent and
+        //     never rejecting, so it can never block the checkpoint.
+        let handoff = bound_handoff_summary(handoff, HANDOFF_SUMMARY_MAX_CHARS);
+
         // 4. Persist the advanced handoff (overwrites the previous resume point).
         self.handoffs.save(conversation, handoff).await?;