Planning: close M3 and record active-head architecture

M3's review recommended planning changes and, following the M2 pattern,
reported rather than applied them. This applies them, and adds the ADR the
review asked for.

ADR 012 records the architecture rather than the requirement. ADR 005
already says that going backward must preserve abandoned history and that
the user sees Undo/Redo/Retry rather than branch management; it names a
movable active head as the direction and stops. What M3 settled is the
shape: the head is stored rather than derived, every read of the story is
capped at it in one place, one mechanism moves it, the state of a position
comes off the node rather than from a replay, the first write below a
moved-back head is the divergence, and whether Redo exists is decided by
the lineage rather than by a flag that could be stale. The last of those
is the property worth keeping — a flag can be wrong and make the story
wrong; a lineage cannot.

Two semantics are ratified in STORY-BRANCH-SEMANTICS.md, both of them
reversals or narrowings that a reader would otherwise take for bugs. Undo
now crosses fork points and continues to the campaign opening, because
refusing at the fork was a consequence of deleting rows the parent line
was also reading, and nothing is deleted any more. And the system refuses
to switch which take is live while a later story is off screen, because
doing it quietly would leave retained history continuing from words the
story no longer says.

A new §14A covers editing in place. §14-15 describe the finished
behaviour — the edit becomes authoritative, the state it implies is
re-evaluated, a new continuation is created, the original is retained —
and that requirement is intact and explicitly not weakened here. It is
also not built, because re-evaluating state from prose a user typed needs
M5's extraction pass. §14A says what exists in the meantime and why
refusing is the minimum that holds the invariant rather than the
destination.

TECHNICAL-DESIGN.md gains §8.7 and §9.1, recording the implemented model
and the bundle behaviour as fact in the way §5.2 records M1 and M2. §10.4
gains a constraint that is easy to lose: the snapshot half of the hybrid
state model is a requirement, not an optimization. Head movement is a row
lookup plus a restore, which is why Undo, Redo and Save Point restore cost
the same at any distance into a campaign; a state model recoverable only
by replaying from the opening would make all three proportional to
campaign length, on exactly the long campaigns this product is for.

DATA-MODEL.md records the head as stored on the campaign rather than
derived from its newest turn — two campaigns holding identical turns can
be read at different places, and nothing about the turns can tell them
apart — and the branch disposition as implemented: the depth a divergent
write left the branch at, deliberately advisory, and carried through
export because every row of an abandoned line is exported either way.

BUILD-MILESTONES.md marks M3 complete and states the one condition still
open. M4 is told a Save Point is a durable pointer and that restoring one
is head movement with a bounds check, not a restore system: a second
mover is the specific failure to avoid, because the two paths would
silently disagree about what restore means. M5 gets three constraints —
keep state efficiently recoverable, move the test instrumentation rather
than the assertions when the world-state protocol goes, and finish the
narrator edit §14A defers.

V1-ACCEPTANCE-TESTS.md clarifies ownership without lowering a bar. D10
keeps all three pass conditions and is explicitly recorded as *not*
satisfied at the end of M3; what changed is that the document now says
which milestone delivers which condition. D03's result is recorded as a
full pass rather than the partial the text allowed for, I07 gains the
pre-M3 bundle clause, and L01 gains the note that resolves its apparent
conflict with A05 — a failed turn does advance the head by one, onto the
player's retained input, and that is A05 working rather than L01 failing.

README.md described a different application: a hosted demo, guest
accounts, cloud providers, Postgres, a Render blueprint, an analytics
dashboard, a QuickJS scripting engine, and 549 tests. M2 removed all of
that and the README was never updated — a gap M2's own debt table missed.
It now describes what this fork is, including the endpoint policy and the
TLS behaviour, and the numbers in it are the current ones.

M3's report is included here as its own evidence record: no separate
baseline report was produced, so it carries the raw counts and runtime
observations as well as the review, and §W records this closeout.

SPECIFICATION.md and SECURITY-THREAT-MODEL.md are unchanged. M3 altered no
product requirement and touched no path in the threat model.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QF5TcoB86QADgjHz1GZe8u

planning/DATA-MODEL.md

@@ -74,6 +74,14 @@ story_profile:
   tense: optional string
 ```
 
+The active head is **stored on the campaign, not derived** from its newest turn.
+This is the concept M3 implemented; the current implementation carries it as a
+branch reference plus a depth on that branch rather than as a turn id, which is
+an equivalent coordinate and is what the export format records. What matters
+conceptually is that the position is a decision the campaign remembers: two
+campaigns holding identical turns can be being read at different places, and
+nothing about the turns themselves can tell them apart.
+
 Campaigns also store durable narrator rules and model configuration.
 
 Potential model roles:
@@ -109,7 +117,26 @@ Rules:
 - Redo moves the active head forward while the prior continuation remains selected,
 - a new write below the retained tip creates a new continuation and leaves the old future retained/disposable.
 
-Detailed behavior will be defined separately in `STORY-BRANCH-SEMANTICS.md`.
+`disposition` above is conceptual. As implemented in M3 it is stored as **the
+fact that produced it** rather than as a word: a branch records the depth a
+divergent write left it at, and when. No value means active; a value means the
+story past that depth is retained history no active head is reading. The
+shallowest departure wins if a branch is left more than once.
+
+Two properties of that representation are deliberate and worth carrying in this
+document:
+
+- **Nothing reads it to decide behavior.** Whether Redo is available, what the
+  transcript shows, and which continuation a write belongs to are all decided by
+  the lineage. A stale or hand-edited disposition therefore cannot make the story
+  wrong; it can only mislead a cleanup or recovery feature about what is
+  abandoned.
+- **It survives export and import.** Every row of an abandoned line is exported
+  either way, so the disposition is the only thing distinguishing it from an
+  active one in a restored campaign.
+
+Detailed behavior will be defined separately in `STORY-BRANCH-SEMANTICS.md`;
+the architecture is recorded in ADR 012.
 
 ## 6. Turn
 
@@ -655,6 +682,15 @@ A campaign export should be capable of preserving:
 
 The physical container format remains an implementation choice, but the export must preserve the exact active branch **and active head position**, even when the head is behind a retained tip after Undo. A ZIP containing a database plus manifest remains a strong candidate.
 
+As implemented in M3, the export carries the active branch, the active head
+position on it, and each branch's disposition, alongside the whole retained turn
+graph. The governing rule for this package is that an export carries what was
+*chosen* and recomputes what is *derived* — and the active head moved from the
+second category to the first, because once Undo stops deleting, two campaigns
+with identical turns can be being read at different positions and no import can
+tell which. An export written before the field existed is opened at its retained
+tip, which is the position such a file recorded.
+
 ## 30. Deletion vs Archival
 
 The system must distinguish: