Let a backup carry a story that went two ways

A bundle had one list and a forked adventure has two stories, so export was
emitting every branch's turns interleaved by index — a mangled story rather
than lost data, and unreachable only because forking has no UI yet.
`ai-dnd-adventure-v2` carries the branches, the depth each one left its
parent at, which attempt at every turn is the story, and what each node
left behind. That last one is not decoration: the after-snapshots are what
a branch switch puts back, and a bundle without them imports a tree nobody
can switch inside.

`app/bundle.py` owns both formats and nothing else knows either. The v1
reader stays — those files are already on people's disks — and it is now
the only place a `variants` array exists anywhere.

The rule the module is built on is that a bundle carries what was chosen
and never what is derived. The head branch, the fork points, the live flags
and the anchors are decisions somebody made. The lineage, the head depth,
the legacy `index` and the variant ordinals are computed from those and are
rebuilt on the way in, because a bundle is a text file anybody can edit and
a derived field shipped beside its source is a chance for the file to
disagree with itself where no read would report it.

`index` is the one that stops being academic here. It agreed with `depth`
until SP5, and this is the first writer that has to fill it for a forked
story, where two branches both hold a node at depth 4. It is allocated one
per turn instead: siblings share it, no two coordinates do.

Everything a hand-edited file can get wrong about the shape of a tree is a
400 raised before the adventure row exists, because a half-applied import
is exactly the failure this phase exists to end — a story that goes quiet.
A file wrong about which attempt is live is corrected rather than refused;
that is an invariant of the database, not of the format.

Measured on the 600-action fixture: 587 kB to 911 kB, and all of the
increase is the outcomes at 489 B a node — the coordinates themselves save
57.5 B a node against the old turn-and-variants shape. Twenty forks add
660 B. 4.3% of the import body cap.

381 tests green, 16 of them new in test_bundle_v2.py. No migration, no
vacuum owed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015H5qiyiR7gtFQaoDphHZ3g

plan/14-phase-story-tree.md

@@ -37,8 +37,10 @@ existed for undo and delete; see the trap note below. Two rows want a footnote:
   stale exactly as before. What *has* changed is that the machinery to fix it now exists
   — an edit could write a sibling and switch to it, which is a retry the player typed —
   so it is a small change whenever it is wanted.
-- **The 1NF violation** is resolved in the database. The `variants` array survives in
-  exactly one place: the v1 export bundle, which SP6 replaces.
+- **The 1NF violation** is resolved. Nothing writes a `variants` array any more, in the
+  database or out of it — SP6 replaced the bundle that was its last producer, and the
+  array survives only in the v1 *reader*, which exists so files already saved still
+  import.
 
 ## Design decisions (settled 2026-08-16)
 
@@ -610,6 +612,88 @@ bundle's linear actions plus `variants` onto one branch with siblings.
 **Verify:** v2 round-trip of a branched adventure is lossless; a v1 bundle still imports;
 a bundle claiming more branches than rows is rejected rather than half-applied.
 
+**Done, 2026-08-18** (branch `sp6-bundle-v2`). **381 tests green**, the 365 SP5 finished
+with plus 16 in the new `test_bundle_v2.py`. `app/bundle.py` owns both formats; the two
+endpoints in `routers/adventures.py` are a delegation and the shared plumbing, and the
+`variants` array now exists nowhere but the v1 *reader*.
+
+**The rule the module is built on: a bundle carries what was *chosen*, never what is
+*derived*.** The head branch, the fork points, the live flags and the anchors are
+decisions somebody made, and they are in the file. `lineage`, the head *depth*, `index`
+and the variant ordinals are computed from those and are rebuilt on the way in. That is
+not tidiness — a bundle is a text file anybody can edit, and every derived field shipped
+beside its source is a chance for the file to disagree with itself in a way no read would
+report. It is also the answer to "is the round trip lossless?": everything omitted is
+reconstructed, and the tests assert the reconstruction rather than the bytes.
+
+Six things worth not rediscovering:
+
+- **`depth` cannot be the legacy `index`, and this is where that stops being academic.**
+  They agreed until SP5, and a bundle is the first writer that has to fill `index` for a
+  *forked* story — where two branches both have a node at depth 4. `index`'s one
+  remaining job is handing the next row a number nothing else holds, which is a fact
+  about the adventure rather than about a path, so the import allocates one per turn in
+  bundle order: siblings share it, the way SP4 leaves them, and no two coordinates do.
+- **Validation happens before the adventure row exists.** Everything a hand-edited file
+  can get wrong about the shape of a tree — a node naming a branch that is not listed, a
+  fork with no depth, a branch forking from one listed after it — is a 400 raised by
+  `plan`, which touches no session. The alternative is an adventure holding a story with
+  a hole in it, and this whole phase exists because a story with a hole in it fails by
+  going quiet.
+- **A branch may only fork from one listed before it.** That is how the export writes
+  them, and requiring it buys acyclicity for the price of a comparison — a cycle in the
+  parent chain would be an import that never returns rather than one that fails.
+- **`{}` and absent are different snapshots.** An empty `state_after` means "this node
+  left an empty scoreboard behind"; a missing one means "nobody knows, leave the live
+  state alone" (`attempts.restore_state`). Trimming empty dicts on the way out would have
+  saved eighteen bytes a row and turned an undo that clears a score into one that leaves
+  it standing. Only `worldDelta`, which is display, is dropped when empty.
+- **A file is allowed to be wrong about which attempt is live, and the import corrects it
+  rather than refusing.** "Exactly one sibling in a group is live" is an invariant of the
+  *database*, not of the format; a coordinate with none is a turn no read can see, so the
+  first attempt is made live. That is a different class from a missing branch, which is
+  structure, and is refused.
+- **`limits.MAX_BRANCHES_PER_ADVENTURE` (1000) has no live counterpart.** Forking is a
+  POST that adds one row and has no cap of its own, so this is the one bundle cap that
+  does not mirror something creation enforces. Worth closing if branch management ever
+  makes forking cheap to repeat.
+
+**The verify line above was slightly wrong, and the code does the honest version.** "More
+branches than rows" fails on an adventure with no actions, which legitimately has one
+branch and no rows. It became two rules instead: a cap on the branch list, and *every
+branch a node names must exist*.
+
+Measured with `tools/measure_bundle.py` on the 600-action `--rich` fixture — 600 turns,
+750 nodes, because 150 of them were retried:
+
+| | bytes | vs v1 |
+|---|---|---|
+| v1 shape | 587,475 | — |
+| **v2** | **911,229** | **1.551×** |
+| v2 without the outcomes | 544,318 | 0.927× |
+
+**The tree is free; the outcomes are what cost.** Coordinates *save* 57.5 B a node
+against v1's turn-and-variants shape, and the entire 1.55× is `state_after` /
+`world_state_after` at 489 B a node — which are there because a bundle without them
+imports a tree nobody can switch inside. Twenty forks add 660 B to the same file, **33 B
+a branch**, so the format is as indifferent to fork count as the reads are. The longest
+adventure production holds exports at 4.3 % of `MAX_IMPORT_BODY_BYTES`.
+
+**Two lines of the SP0 baseline changed, not the one it predicted.** The note in
+`test_story_tree_baseline.py` allowed for the `format` assertion; the `variants` array in
+`test_export_keeps_retry_attempts` is the same fact from the other side — a bundle with
+coordinates has no use for a repeating group. Everything else in that file still passes
+unmodified.
+
+**No migration, no vacuum.** SP6 adds no column and rewrites no row.
+
+**And one trap, paid for once.** `tools/measure_bundle.py` imported `app` before
+`tools.stress_session`, which is what points `AIDND_DB_PATH` at a throwaway file —
+`app.database` reads it at module scope. It fails by *working*: the first run seeded a
+synthetic user and adventure into the local `backend/data.db` and printed perfectly good
+numbers, and only the second run tripped over the unique email. Anything importing that
+harness must import it first, and the file now says so where the imports are.
+
 ### SP7 — Frontend: full tree visualisation
 
 `VariantPager` is removed. A spatial tree view replaces it, plus switch, rename and