docs(planning): record horizon and cancellation decisions (#12)

CONTEXT.md

@@ -25,7 +25,11 @@ The category of leave in a balance: CP (paid leave), RTT (working-time reduction
 _Avoid_: ANY as a leave type; `ANY` is only a rule selector.
 
 **Planning horizon**:
-The user-selected half-open date interval of at most 12 months within which leave may be placed. It defaults to today's Europe/Paris calendar date through the same date 12 months later.
+The user-selected half-open date interval within which leave may be placed; its start may precede today's Europe/Paris calendar date and its duration may exceed twelve months. It defaults to today through the same date twelve months later, and explicitly selected boundaries remain unchanged as time passes, with past dates still eligible for leave placements.
+
+**Last planning date**:
+The inclusive final calendar date selected by the user for leave placement; the planning horizon's exclusive end is the following calendar date.
+_Avoid_: horizon end
 
 **Working day**:
 A Monday-to-Friday calendar date that is not a French national public holiday. Only working days can receive leave placements.
@@ -132,7 +136,7 @@ The untruncated phase-by-phase record captured by the calculation that produced
 A hypothetical comparison that keeps the calculation snapshot and all unrelated placements fixed while testing one selected placement across every date in the half-open planning horizon. It neither mutates the plan nor rewrites the original trace.
 
 **Incomplete calculation**:
-A calculation stopped before it completed the planning objective order. It may expose its best valid plan but never implies that the obligations conflict.
+A calculation stopped by user cancellation or another interruption before it completed the planning objective order. On user cancellation it exposes its best available valid plan as partial, if one exists, and never implies that the obligations conflict.
 
 **Planning objective order**:
 The lexicographic quality order applied after joint obligation feasibility: minimize unplaced at-risk leave; satisfy each preference in user order; maximize public-holiday week value; maximize non-working-day gain; minimize the longest work sequence; minimize the total deficit of internal work sequences shorter than five days; minimize supporting placements; consume earlier-expiring balances; then prefer earlier placement dates. Complete holiday weeks therefore precede broader calendar-gain and work-sequence optimization. A lower criterion never degrades a higher one.