feedback1 WE: 鸭爵 swaps stay on the player's half of a pair leader field

- A pair Final Assault / Hidden Core field has one spawn list that both
  players' handlers edit; each player's 0-2 swaps now come only from the
  enemies whose route ends on that player's half (the half rule of the
  field's bounties). Match passes the wave's routes and the player's side
  ('L' / 'R', null on a one-player field) in the boss / hidden
  onBattleStart event; docs/META.md documents the two fields.
- The comment justifying elite eligibility was wrong (the leader rounds'
  escorts are a mix, about two thirds elites, not all elites); reworded
  with the measured numbers and kept [ASSUMED].
- The pair-field test now drives the real match path (Final Assault, then
  the Hidden Core it unlocks) instead of dispatching by hand; a unit check
  covers the half filter.

docs/META.md

@@ -246,7 +246,7 @@ Every handler method is `(ctx, ev)`; `ev` is shared by all handlers of one dispa
 | `onMerge` | chess or item merge | `{ kind, piece, baseId|itemId, consumed:[uid], area? }` — a chess merge's `area` is where its elite went: 'board' (a consumed copy's tile) \| 'hand' \| 'temp' |
 | `onLevelUp` | shop level up | `{ level, price }` |
 | `onSpend` | a payment's action is complete (buy / refresh / levelUp / reward / effect) | `{ amount, reason, total }` (`total` = funds spent this match) |
-| `onBattleStart` | a battle input is built (normal / unite / boss / hidden); also for the stats preview (`g.unitStats`, DESIGN §18.5) | `{ input: PlayerBattleInput, kind, round, spawns?, preview? }` — mutate/replace `ev.input`. With `ev.preview` true (no `ev.spawns`) a handler must NOT change match state or draw match rng: the preview only reads the input |
+| `onBattleStart` | a battle input is built (normal / unite / boss / hidden); also for the stats preview (`g.unitStats`, DESIGN §18.5) | `{ input: PlayerBattleInput, kind, round, spawns?, routes?, side?, preview? }` — mutate/replace `ev.input`; `ev.spawns` (the field's spawn list, shared by a boss field's players) may be edited in place. Boss / hidden fields also carry the wave's `routes` and the player's `side` ('L' / 'R' on a pair field — its half is the routes ending left / right of the middle column — null on a one-player field), so a handler can edit only the player's own enemies (鸭爵's swap). With `ev.preview` true (no `ev.spawns`) a handler must NOT change match state or draw match rng: the preview only reads the input |
 | `onBattleResult` | SETTLE (normal rounds) and after boss fields | `{ result, lpLoss, perfect, unite?|boss? }` |
 | `onChoicePick` | a 机变 card was applied (the card's own `choice:` handler runs first, then every source observes) | `{ card, family, picker, forTeammate }` |
 | `onEquip` | `g.equip` (item handler only) | `{ item, target, golden, consumed, keep, error }` — see §2.5 |