M4: add durable named Save Points

A Save Point is a name for a story position, and restoring one is head
movement. That is the whole architecture, and it is what ADR 012 and
BUILD-MILESTONES' note on M4 asked for: M3 made the head a stored
(branch, depth) and made arriving at one a row lookup plus a state restore,
so a Save Point needs no restore machinery of its own.

What the user gets:

- Name the moment they are reading, keep playing, restart the app, and come
  back to it. Restoring moves the story back and deletes nothing: the later
  turns stay, Redo still walks forward into them, and writing something
  different is what starts a new line while the old one is kept.
- Rename, delete, and a list, in a Save Points panel beside the branch panel,
  with a Save Point button next to Undo and Redo. Both confirmations say what
  is *not* destroyed, because that is the part the screen cannot show.
- Save Points survive export and import.

What was deliberately not built:

- No second restore path. `head.move_to_node` is the only new movement: its
  depth half is M3's `head.move_to` unchanged, and its branch half is the
  single assignment `switch_branch` already makes. No head field is written
  in the checkpoint router, nothing reconstructs state, nothing prunes a
  memory, nothing copies or deletes a turn, and restore never forks — the
  first write below the restored head does, through `fork_if_behind_head`.
- No automatic cleanup. A Save Point behind the head, or naming a line the
  story left, is doing its job (STORY-BRANCH-SEMANTICS §19). The one removal
  is a cascade: deleting a branch takes its Save Points, as it takes its
  memories, because the story they named went with it.
- No new ADR. ADR 012 already decides the architecture, and a table is not a
  decision.

The one call the planning package did not already make: restore moves the
branch half of the head only when the coordinate is off the path being read.
Doing it unconditionally would quietly hand back an abandoned continuation
whenever a Save Point in a shared prefix was restored; never doing it would
make a Save Point on a departed line unrestorable, which contradicts §19.
TECHNICAL-DESIGN §8.8 records it.

Schema: a `checkpoints` table holding a name, an optional note and a
(branch, depth) coordinate — no copy of any story. `create_all` builds it as
it did `memories` and `branches`; migration 80 adds the index. No backfill,
because nobody had named a position before M4.

The coordinate is deliberately not an action id: one coordinate holds every
attempt at a turn and exactly one is live, so a coordinate follows a retry
where a row id would pin a take the story no longer tells.

Tests: 680 pass (638 before). 42 new in tests/test_save_points.py covering
D11-D14, I04, L03, E-series lineage and memory isolation after restore and
divergence, the edge cases, and an M3-database migration. One pre-existing
fixture in test_tree_migration.py needed `checkpoints` added to its drop
list — SQLite refuses to drop a table another table references.

Not verified: the browser. No session has had a usable one, so the Save
Point panel's DOM behaviour is unobserved — as M3's Redo control still is.
The twenty-step sequence was driven over HTTP against a live server with a
real process restart instead, and all seventeen checks pass. M4 is
implemented, not accepted: no review has been written.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PWU4gTfLYY6Qq9U7aa9Qw2

README.md

@@ -67,10 +67,20 @@ that isn't the live one starts a new branch.
   displaced future stays on the line it was written for, and ordinary Redo stops offering it.
   Nothing a retry replaces is discarded either — the old attempt stays as another take of that
   turn, one keystroke and one click from becoming a branch of its own.
+- **Save Points.** Name a moment — "Before entering the abbey" — keep playing,
+  restart the app, and come back to it. Restoring one moves the story back to
+  that moment and deletes nothing: the turns you wrote after it stay, Redo still
+  walks forward into them, and writing something different from the Save Point
+  is what starts a new line while the old one is kept. A Save Point is a name for
+  a position and holds no copy of the story, so restoring it is the same
+  movement Undo makes (`backend/app/routers/adventures/checkpoints.py`,
+  `backend/app/head.py`). They last until you delete them, and deleting one
+  deletes no story.
 - **Import and export.** AI Dungeon-compatible scenario format; JSON for everything else. An adventure exports as `ai-dnd-adventure-v2`, which carries the whole tree:
-  every branch, every take, the fork points, which branches the story has left behind, and the
-  position it is being read at — all of them chosen rather than computed, which is the rule for
-  what a bundle carries. A campaign exported after two Undos imports still undone, with its
+  every branch, every take, the fork points, which branches the story has left behind, the Save
+  Points and the position it is being read at — all of them chosen rather than computed, which is
+  the rule for what a bundle carries. A campaign opens where its head says, never at a Save Point
+  merely because it has one. A campaign exported after two Undos imports still undone, with its
   retained future intact, instead of silently reopening at its newest turn. Files that predate
   the head position, and files saved in the old single-line format, still import.
 - **Single user, no accounts.** There is no sign-up, no login, no session and no API key
@@ -196,6 +206,7 @@ frontend/   React + Vite SPA  ──HTTP/SSE──►  backend/  FastAPI
                                               ├─ tlstrust.py   one TLS context: the OS trust store unioned with certifi's
                                               ├─ tree.py       forking, promotion, and where a node is placed
                                               ├─ head.py       the active head: where the story is read, and what moving it costs
+                                              ├─ checkpoints   Save Points: durable names for positions, in routers/adventures/
                                               ├─ attempts.py   the takes of one turn, grouped by parent
                                               ├─ context/      prompt assembly under a token budget + lineage/history windowing
                                               ├─ worldstate/   the stat engine: clamps, cooldowns, bands, milestones
@@ -210,7 +221,7 @@ development, Vite proxies `/api` to FastAPI.
 
 ## Tests
 
-638 backend tests: unit tests plus full HTTP integration through the real turn engine, with
+680 backend tests: unit tests plus full HTTP integration through the real turn engine, with
 the model provider mocked. They run with no route to the Internet, which is a requirement
 rather than a convenience — an offline claim proved on a machine that has been online once
 proves nothing.