Stop paying twice for a block a retry can still throw away

A memory whose block ends on the newest action is the one memory a player
can reach: retry and take-switching both refuse anything else. Each retry
of that turn withdrew the memory and wrote it again, and a block closes
every six actions while a normal turn writes two, so that was one turn in
three.

SETTLE_SLACK asks for one action past a block before the block is
summarized. The block is still MEMORY_INTERVAL actions; only the moment
moves. This is not the pre-SP4 holdback returning: that one was about a
retry rewriting text in place, which sibling attempts and forget_node
settled, and correctness still rests on the withdrawal rather than on the
slack. Undo and delete can carry a summarized node back to the tip, so the
withdrawal path stays reachable, just rarely.

The slack buys nothing back. The block that just closed is still in the
history window in full, so a memory of it says what the model can already
read.

Tests: the settling suite asserts the new rule and that a retry at the tip
finds nothing to withdraw; the rewrite suite builds thirteen actions so
both of its blocks settle; the one path test that ended on a block boundary
sets the slack to zero, because it is about which actions a block is read
from rather than about when a block forms. 632 green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015NcrxCJjqgDvAkamKeLWdn

plan/STATUS.md

@@ -76,6 +76,44 @@ needed; nothing requires reading a row of anyone's story.
 
 ---
 
+## What happened on 2026-08-31, part two — one action of settling slack
+
+**A block is no longer summarized while it ends on the newest action.**
+`memorybank.SETTLE_SLACK` (1) asks for one action past a block before the block is
+written. The block is still `MEMORY_INTERVAL` actions; only the moment moves.
+
+**This is not the SP4 holdback coming back, and the reasoning is worth keeping
+straight.** The holdback existed because a retry rewrote `Action.text` in place, so a
+memory could describe narration that no longer existed and nothing reported it. Sibling
+attempts plus `forget_node` settled that, and SP4 deleted the holdback on the grounds
+that "nothing is left to hold back". True — about staleness. Nobody priced the other
+half. Withdrawing a memory and writing it again is correct and it is also two calls for
+one block.
+
+**Retry and take-switching both refuse anything but the newest action**
+(`takes.retry_action`, `takes.switch_take`), so the exposure was exactly one memory: the
+one whose block ends on the tip. A block closes every 6 actions and a normal turn writes
+2, so that was one turn in three, and each further retry of that turn paid again. The
+summary pass was never the expensive half — `rewind_all` takes the mark to the same
+depth however many times a player retries.
+
+**The slack buys nothing back, which is why it is worth having.** The block that just
+closed is still inside the 16384-token history window in full, so a memory of it tells
+the model what it can already read. Memories earn their place once the raw text has
+scrolled out, and that is never the turn the block closed.
+
+**`forget_node` stays, and its path is still reachable.** Undo and delete reach any node,
+and either can carry a summarized node back to the tip, where a retry of it lands on the
+withdrawal again. Rare now instead of routine.
+
+632 tests green. Three test files moved with it: `test_memory_settling.py` asserts the
+new rule (and a retry at the tip finding nothing to withdraw), `test_memory_rewrite.py`
+builds 13 actions instead of 12 so both of its blocks settle, and the one path test in
+`test_memory_nodes.py` sets the slack to 0, because it is about which actions a block is
+read from rather than about when a block forms.
+
+---
+
 ## What happened on 2026-08-31 — the persona, and what the summarizer is told
 
 **`plan/18-persona-and-memory-quality.md` is the writeup. Both changes are on `main`**