docs: initiative ordering design (slot, never sort)

Single list = display = turn order. Slot by initiative, tie-break = add
order. Drag = same-init only (DM tie override). No re-sort on mutation.
Round wrap no rebuild. No cross-init drag.

docs/INITIATIVE_ORDERING.md

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+# Initiative Ordering — Design
+
+## Core Principle
+
+**Slot, never sort.**
+
+Initiative order = a single list. Participants occupy slots determined by
+initiative value. The list is mutated by insert/move operations — never
+re-sorted wholesale.
+
+## Single Source of Truth
+
+One list: `participants[]`.
+
+- Display order = `participants[]` order (both Player view + DM view).
+- Turn order = `participants[]` order.
+- No derived/re-sorted copy. `turnOrderIds`, if present, is always an exact
+  mirror (`participants.map(p => p.id)`) — never an independent ordering.
+
+## When Ordering Changes
+
+| Action | Effect on order |
+|--------|-----------------|
+| **Add** (roll or manual) | Insert participant into slot by initiative |
+| **Edit initiative** | Move participant to new slot |
+| **Drag** | Reorder within a tie (same initiative) only |
+| **Remove** | Splice out; others keep position |
+| **Start encounter** | Freeze current list. No re-sort. |
+| **Round wrap** | No rebuild. Continue rotation through existing list. |
+| **Damage/heal/death/save** | No order change. |
+| **Toggle active** | No position change. Skip in rotation only. |
+
+## Slotting Rules
+
+- Insert/move positions participants so the list stays initiative-descending.
+- **Tie-break = original add order.** Later additions slot *after* existing
+  same-init participants. Stable insertion.
+- Tie order is only changed by explicit DM drag. Never auto-changed.
+
+## Drag (DM Tie-Break Override)
+
+- Only participants with the **same initiative** are draggable onto each other.
+- Cross-initiative drag is **not allowed**. (Use edit-initiative field instead.)
+- Drag persists in `participants[]`. Survives all subsequent operations.
+- **Re-slotting on add/edit must preserve drag-established tie order.**
+
+## Explicitly Forbidden
+
+- `sort()` on every mutation. Overwrites drag order. Root cause of past drift.
+- Separate display order vs turn order. Causes player/DM divergence.
+- Re-sort on round wrap. Replays rounds, introduces skips.
+- Cross-initiative drag. Contradicts initiative as the primary key.
+- Auto-changing tie order on add/edit. Silently loses DM intent.
+
+## Why
+
+Sorting is destructive to manual ordering. A stable slot list with drag for
+ties gives deterministic, DM-controllable order that never drifts between
+display surfaces or combat rounds.