feat(progression): Playthrough aggregate (slice 3)

no LoseLife (ADR-0005). SettleDeath / SettleCompletion defer to the
Attempt's terminal outcome, so both are atomic and idempotent.
SettleCompletion branches on HasCleared itself - a Replay cannot bank
or move campaign position backwards (ADR-0003).

Campaign value object holds authored level order; ReachableLevels is
derived, replacing the stored UnlockedLevels. Coins.IsAtLeast driven
out by cross-scope affordability.

29 tests. CONTEXT.md: Campaign, Settle.

CONTEXT.md

@@ -28,6 +28,12 @@ _Avoid_: Global state, meta, persistent data, account
 
 ### Operations
 
+**Settle**:
+Resolving an ended Attempt against the Playthrough — Commit it on a First Clear, Discard it otherwise. Settling is
+the only way an Attempt affects a Playthrough, and it decides First Clear versus Replay itself rather than being
+told which one happened.
+_Avoid_: Apply, resolve, finalise
+
 **Commit**:
 Applying an Attempt's ledger to the Playthrough. Only a First Clear commits — nothing else, ever.
 _Avoid_: Save, persist, flush, sync
@@ -59,6 +65,12 @@ One playable stage, identified by a stable `LevelId` — never by position in a
 name. Ids must survive inserting and reordering levels, because Playthroughs and Best Times both reference them.
 _Avoid_: Stage, scene, level index
 
+**Campaign**:
+The authored ordering of Levels that defines progression — which Level is first, and what follows a given one. One
+Campaign is shared by every Playthrough: it is authored data, not per-Playthrough state, and it carries no scene
+or presentation detail.
+_Avoid_: Level list, level order, world, chapter
+
 **Cleared Levels**:
 The Levels a Playthrough has completed at least once. Gates both campaign progression and level select; whether a
 Level is reachable is computed from this, never stored alongside it.