docs: archive parent_step_no design plan

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/plan_parent_step_no.md

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+# Design: `parent_step_no` Column on Steps Ntuple
+
+## Problem
+
+The Steps ntuple records `parent_id` (which track created a secondary) but not *at which step* of the parent the secondary was born. Finding this after the fact requires a position join: match the secondary's first pre-step point against the parent's post-step points across ntuple rows.
+
+## Proposed solution
+
+Add a `parent_step_no` integer column to the Steps ntuple. Value is `-1` for the primary particle and the parent's step number for every step of any secondary.
+
+## Mechanism
+
+Use `G4VUserTrackInformation` to carry the parent step number forward from creation time to the secondary's tracking phase:
+
+1. **Tag at creation** — in `UserSteppingAction`, iterate `step->GetSecondaryInCurrentStep()` and attach a `TrackUserInfo(track->GetCurrentStepNumber())` object to each secondary via `const_cast<G4Track*>(sec)->SetUserInformation(...)`. Geant4 takes ownership and deletes it with the track.
+
+2. **Read at tracking** — when filling a Steps ntuple row, retrieve `dynamic_cast<TrackUserInfo*>(track->GetUserInformation())` and write `info->parentStepNo`, or `-1` if null (primary).
+
+## Files
+
+- `include/TrackUserInfo.hh` — new: minimal `G4VUserTrackInformation` subclass storing `int parentStepNo`
+- `src/RunAction.cc` — add column 27 `parent_step_no` (Integer) to ntuple 1 (`Steps`)
+- `src/SteppingAction.cc` — tag new secondaries; fill column 27
+
+## Note on `const_cast`
+
+`GetSecondaryInCurrentStep()` returns `const G4Track*`. The `const` is a conservative API choice, not a physics invariant. `G4VUserTrackInformation` is an explicit user-side side-channel; using `const_cast` here is standard Geant4 practice.
+
+## Superseded by
+
+This approach was superseded by the `Spawning` ntuple (branch `spawning-ntuple`), which avoids the per-row overhead and `const_cast` by pre-assigning track IDs in `G4SteppingManager` and writing a dedicated secondary-birth table.