## Summary
Reshape the speculation tree data model around the Base/Head path that
the build stage actually consumes, and align its store with the
speculation design.

entity: SpeculationPath{Base, Head} becomes the unit;
SpeculationPathInfo carries the path plus its Score, controller-owned
Status
(candidate/selected/prioritized/building/passed/failed/cancelling/cancelled),
and BuildID — Path is immutable once persisted, Score/Status/BuildID are
updateable by the controller. Score is scorer-computed and
controller-persisted dynamic state — recomputed on every respeculate,
not set at enumeration. Selected/Prioritized and Cancelling/Cancelled
split desire vs budget-cleared and cancel-intent vs terminal, so the
persisted status carries the cross-stage contract. Adds
SpeculationPathAction (Promote/Cancel) and SpeculationPathDecision for
the selector and prioritizer seams. SpeculationTree.Speculations becomes
Paths, and the tree gains a Version for optimistic locking (starts at 1,
incremented per change). The Build entity uses the shared
SpeculationPath (Head = the batch under verification) and drops its own
Score, which was never populated or read.

storage: SpeculationTreeStore.UpdateSpeculations(batchID,
[]SpeculationInfo) becomes a pure conditional write — Update(ctx,
batchID, oldVersion, newVersion, paths) — returning ErrVersionMismatch
when the persisted version does not match, per the repo's
optimistic-locking pattern (version arithmetic owned by the controller).
The MySQL impl treats rowsAffected == 1 as success, 0 as version
mismatch, and anything else as a generic error. The MySQL column
speculations is renamed paths and the schema gains a version column.
MySQL impl, mock, and storage integration coverage (create/get
round-trip, duplicate, versioned whole-tree overwrite, stale-write
rejection, not-found) added/updated.

## Test Plan


## Issues


## Stack
1. @ #231
1. #315
1. #316
1. #317
1. #320