refactor(planning): remove the original decision trace

CONTEXT.md

@@ -132,23 +132,8 @@ The numerical summary of a calculated valid plan's leave usage, time-off periods
 A stable account of why an optimization result or incomplete calculation contains its displayed leave placements, expressed through placement provenance, planning objectives, time-off periods, and remaining balances. It reports every contribution of a placement while highlighting the first planning objective that made its position decisive, and describes the displayed plan rather than the calculation history.
 _Avoid_: calculation explanation
 
-**Calculation trace**:
-The complete, untruncated, ordered history of the planning decisions and candidate evaluations from the same calculation that produced a planning outcome, grouped by calculation phase. It is not reconstructed by replaying the calculation afterward; for an incomplete calculation it ends at the actual interruption point.
-_Avoid_: plan explanation
-
-**Planning decision**:
-An atomic choice within a calculation phase that accepts, moves, removes, or rejects one or more related leave placements. A joint obligation decision may therefore concern several dates.
-_Avoid_: calendar day, step
-
-**Candidate evaluation**:
-The comparison the planner actually performs between a possible leave-placement change and the current plan. It belongs to the calculation trace whether the candidate is accepted or rejected.
-
-**Candidate date status**:
-The historical relationship between a date and a planning decision: chosen, evaluated and rejected, not examined, or ineligible with a reason. A later counterfactual date analysis never changes this historical status.
-
 **Counterfactual date analysis**:
-An on-demand evaluation that changes only the date of one selected leave placement while preserving its leave balance, leave type, and placement provenance. The planner did not necessarily examine that date; the analysis explains a hypothetical alternative but never influences the planning outcome and is not part of the calculation trace.
-_Avoid_: candidate evaluation, calculation trace
+An on-demand hypothetical comparison of another date for a displayed leave placement. It uses the immutable calculation configuration and final-state snapshot, preserving the same leave entitlement, leave type, and placement provenance while keeping unrelated placements fixed. It never mutates the displayed plan.
 
 **Conflict explanation**:
 A stable account of why no plan can jointly satisfy a set of obligation rules within the planning horizon and available leave balances.
@@ -166,12 +151,6 @@ The result of a calculation: invalid input, a planning conflict, an optimization
 **Stable plan explanation**:
 A deterministic explanation derived from the final displayed plan. It identifies each placement's entitlement and rule provenance, contributions, full objective vector, and the first objective that distinguishes the plan with that placement restored from the same plan with only that placement removed.
 
-**Original decision trace**:
-The untruncated phase-by-phase record captured by the calculation that produced the displayed outcome, including atomic decisions, candidate statuses, and state deltas. It is historical evidence and is never recomputed from the final plan.
-
-**Counterfactual date analysis**:
-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 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.
 
@@ -192,4 +171,6 @@ A valid deterministic plan produced after completing the planning objective orde
 Any quantity left unspent by the optimization result, reported as unavoidable expiry, an unplaced at-risk balance, or a deferred balance.
 
 **Non-placement reason**:
-A concrete explanation of why a quantity of a leave balance remains unspent in the displayed plan, distinguishing a blocking condition from a decision to preserve leave for later. It includes the relevant dates, is distinct from the balance's expiry-based classification, and can be supported by recorded candidate evaluations.
+A concrete explanation of why a quantity of a leave balance remains unspent in the displayed plan, distinguishing a blocking condition from a decision to preserve leave for later. It includes the relevant dates, is distinct from the balance's expiry-based classification and is derived from the displayed plan.
+
+The original decision trace, historical date analysis, historical calendar replay, candidate statuses, and remaining-balance attempt lists were removed by issue #76. Calculations retain only the snapshots needed for outcomes, cancellation recovery, stable explanations, and displayed-placement date analysis.