feedback3 WJ: prep watch of another board updates when it changes

A scout stays subscribed to n:<pid> and gets a new prep field when that board moves, including a spectator. A shop change does not push one.

docs/DESIGN.md

@@ -559,7 +559,7 @@ Audio: autochess BGM per phase, UI SFX (buy/sell/refresh/level/merge/ready/timer
 
 **Solo pause (official `PauseUp` / `ResumeUp`; `Match.setPause`).** C→S `g.pause { on }`: solo matches only (co-op → `WRONG_PHASE 'co-op battles never pause'`), and `on: true` only while a battle runs (COMBAT / FINAL_ASSAULT / HIDDEN_CORE with a live field, not once the Final Assault is ending — else `WRONG_PHASE 'no battle running'`); `on: false` is always accepted. While `m.public.paused` is true everything of the battle is frozen: the field clock (`b.start elapsed`, the boss plausibility budgets and the overtime drain), the authority deadline and the server-run release timers, the boss clock, the HUD `deadline` / `overtimeAt` (shifted by the paused time on resume) and the server-run pacers (FieldRunner / HeadlessPacer skip their intervals); the browser's runner stops its local clock. In solo battles the top bar shows a 暂停 / 继续作战 button (`ui/hud.js PauseButton`, `ui/matchStatus.js pauseAvailable`; `Space` toggles it), a paused overlay offers 继续作战 / 放弃模拟, and the HUD clocks freeze at the pause moment. A disconnect or leave resumes (the server takes the field over — nobody is left to resume it); the battle phase ending drops the pause.
 
-**Spectating (research 09 §3.1 / §6.3).** No live spectating while your own normal battle runs (`g.watch` → `WRONG_PHASE 'own battle running'`; the UI toasts "作战中无法查看队友"). After your own battle ends you see "⌛ 作战结束，等待队友完成作战" with each teammate's live progress (`m.public.fields[].progress`); tapping a teammate's avatar expands **前往查看** → `g.watch` → `b.start` (display replica fast-forwarded to the field clock); the own row shows **返回战场**. Prep: the same flow shows the teammate's board (`m.field` with `prep: true`, unchanged). 联防: every human receives the unite spec (helpers + observers). Final Assault: each human sees its own pair field; the other pair → `BAD_TARGET 'other group hidden'`. In 联防/FA fields the ‹ › pill switches the camera LEFT half / 全景 / RIGHT half ("你自己" / "👁 name" / "全景" / "无人在家"); there is **no** view switcher during normal combat. Eliminated players start each combat phase on the first field (auto-observe) and may watch any field.
+**Spectating (research 09 §3.1 / §6.3).** No live spectating while your own normal battle runs (`g.watch` → `WRONG_PHASE 'own battle running'`; the UI toasts "作战中无法查看队友"). After your own battle ends you see "⌛ 作战结束，等待队友完成作战" with each teammate's live progress (`m.public.fields[].progress`); tapping a teammate's avatar expands **前往查看** → `g.watch` → `b.start` (display replica fast-forwarded to the field clock); the own row shows **返回战场**. Prep: the same flow shows the teammate's board (`m.field` with `prep: true`) and sends it again when that board changes (GitHub #87). 联防: every human receives the unite spec (helpers + observers). Final Assault: each human sees its own pair field; the other pair → `BAD_TARGET 'other group hidden'`. In 联防/FA fields the ‹ › pill switches the camera LEFT half / 全景 / RIGHT half ("你自己" / "👁 name" / "全景" / "无人在家"); there is **no** view switcher during normal combat. Eliminated players start each combat phase on the first field (auto-observe) and may watch any field.
 
 **Client runner.** `public/js/battle/runner.js` owns the local Battles: fixed-step clock at the battle speed (60 ticks per real second at 2×, at most `max(8, 4·speed)` ticks per frame, bounded fast-forward of ≤ 240 ticks per frame when far behind; a new battle is caught up silently before it is shown), feeds the existing renderer through the same `snapshot()`/`drainEvents()` wire formats every frame (`view.setLocalFeed` shrinks the interpolation buffer to ~2 frames), publishes the field meta (m.field shape + `local`, `battleId`, `players`, `sides`, `speed`) into `store.match.field` and its state into `store.match.battle` (with `leaks: { [fieldId]: counted leaks so far }` of every normal field it simulates and, while the 联防 field is on screen, `uniteLeft` per leaker (§20.6), authoritative or display replica — the settle rule's count, `/sim/spec.js battleProgress`; published in the frame a count changes: the live LP of §17.5), sends `b.progress`/`b.result` when authoritative (results retried once on timeout; a result lost with the socket — the request failed `DISCONNECTED` / `OFFLINE` or timed out twice — is kept and sent again when the session is back online and whenever a `b.start` still names that finished battle `authoritative: true`; the server answers a duplicate `ok` and changes nothing; a server refusal is final), follows `m.public.paused` (solo pause: every local clock stands still, resume shifts it by the paused time), handles `b.pool`/`b.end`, fast-forwards on reconnect/observe, and survives tab backgrounding (a 250 ms interval pump keeps authoritative battles on their clock while hidden; the server deadline + takeover cover extreme cases). Catch-up frames keep the state events and every enemy form fx (`keepsState`); while hidden, the battle on screen holds them for the first frame back (`hold`; past `HELD_MAX` the superseded status / skill toggles are dropped, `compactHeld`, and only a backlog still past it re-enters the view from the field meta), and a battle that ended while hidden delivers them when the tab is shown; a catch-up frame or a hidden step drains its events every `EV_SLICE` (15) ticks, each batch stamped with its own game time (`gt`), so a late form fx reaches the render engine as late as it is; the game screen's pre-entry buffer (`keepEarly`, also while a re-sent field meta re-enters the field on screen) and the render engine's event queue (`render/interp.js isCosmeticEvent`) never drop a form fx either (§21.4). Sim content errors are logged as console warnings (the server logs its own). The sim loader fetches the ten data files (one retry each) and fails when any is missing — a battle on partial data would report a plausible wrong result — so the next `b.start` retries and the server's deadline covers the field meanwhile. `public/js/battle/observe.js` holds the observing rules and labels.