feedback5 R5: split the field-view helpers out of the render app

PIXI loading, pick records, tunables, view kind, unit info and the asset / GL
host now live in public/js/render/app/*.js. createFieldView stays in app.js,
unchanged, and every name the file used to export is re-exported. One
pixiPromise. Two source pins follow the lines that moved.

public/js/render/app/view.js

@@ -0,0 +1,95 @@
+// public/js/render/app/view.js — which field a camera shows, and the board box that follows it.
+
+import { AREAS, areaFor } from '../board3d/layout.js';
+import { normRect } from '../projection.js';
+
+const VIEW_KINDS = new Set(['prep', 'normal', 'unite', 'boss', 'bossPrep', 'pen']);
+
+/**
+ * The field a camera request actually shows — what render/projection.js presetCamera frames: 'hidden' → 'boss';
+ * a 'prep' camera asked for the boss rows (Final Assault prep: rect r1 ≤ 6) → 'bossPrep'; unknown kinds → 'normal'.
+ * Drives the built 3D area, the drawn 2D rows and the lit rect, so they always match the camera.
+ */
+export function viewKind(kind, opts) {
+  let k = kind === 'hidden' ? 'boss' : kind;
+  if (!VIEW_KINDS.has(k)) k = 'normal';
+  if (k === 'prep' && opts && opts.rect && normRect(opts.rect).r1 <= 6) k = 'bossPrep';
+  return k;
+}
+
+/** 3D areas without the enemy preview pen block (the own field / both normal halves); see `boardArea`. */
+const AREA_NO_PEN = Object.freeze({
+  normal: Object.freeze(AREAS.normal.filter((a) => a.r1 <= 13)),
+  unite: Object.freeze(AREAS.unite.filter((a) => a.r1 <= 13)),
+});
+
+/**
+ * 3D area built for a view kind (viewKind): the enemy preview pen (rows 14–18) only for the 'pen' camera — the prep,
+ * battle and 联防 cameras show the field alone (user playtest #2 item 6) with its separator rows 6 and 13 (the row-13
+ * devices blow into the field: act2 m01's blowers, user playtest #5 item 6; the boss field's row-6 devices are drawn
+ * with the boss field only — board3d/layout.js stageDevices); the boss kinds build the boss field.
+ */
+export function boardArea(vk) {
+  if (vk === 'pen') return AREAS.normal;
+  if (vk === 'prep' || vk === 'normal') return AREA_NO_PEN.normal.length ? AREA_NO_PEN.normal : AREAS.normal;
+  if (vk === 'unite') return AREA_NO_PEN.unite.length ? AREA_NO_PEN.unite : AREAS.unite;
+  return areaFor(vk);
+}
+
+/**
+ * 2D rows drawn for a view kind: the pen rows (14–18) only for the 'pen' camera; the boss field with the separator
+ * and the normal rows behind it as scenery.
+ */
+export function bandFor(kind) {
+  if (kind === 'boss' || kind === 'hidden' || kind === 'bossPrep') return [0, 13];
+  return kind === 'pen' ? [6, 18] : [6, 13];
+}
+
+/**
+ * Active field rows [r0, r1] of a view kind for the 2D board (render/tiles.js `setView` field: drawn rows outside it are
+ * dim scenery without devices): the normal / 联防 / prep fields live between the separator walls (rows 6–13, the
+ * devices on the row-13 wall included; the row-6 wall's belong to the boss field — tiles.js _stageDevices); the pen
+ * camera adds the pen (6–18); the boss field 0–6.
+ */
+export function fieldRows(kind) {
+  return kind === 'boss' || kind === 'hidden' || kind === 'bossPrep' ? [0, 6] : kind === 'pen' ? [6, 18] : [6, 13];
+}
+
+/** Are the pen's figures shown for a view kind (a camera flight shows them when either end is the pen)? */
+export const penShown = (vk, prevVk = null) => vk === 'pen' || prevVk === 'pen';
+
+/** Is the prep's leader on the boss field shown for a view kind (a flight shows it when either end is the boss-field prep)? */
+export const leaderShown = (vk, prevVk = null) => vk === 'bossPrep' || prevVk === 'bossPrep';
+
+/** '2d' | '3d' | 'auto' board preference: `?board=` in the page URL (dev), else the view option. */
+export function boardPreference(opt) {
+  let q = null;
+  try { q = new URLSearchParams(globalThis.location?.search || '').get('board'); } catch { /* no location */ }
+  const v = q || opt;
+  return v === '2d' || v === '3d' ? v : 'auto';
+}
+
+/**
+ * A battle device box (render/units.js DeviceView `ctx.createBox()` contract: `{ mesh, update(cam, b), destroy() }`)
+ * that follows the board layer: the 3D scene's crate mesh while `board()` returns a BoardScene, else the Pixi box of
+ * the 2D board. A board switch mid-battle (lost WebGL context → 2D, or back to 3D) swaps the inner box on the next
+ * update instead of leaving the crate on a dead scene. `mesh` is the Pixi mesh (null in 3D: nothing to add to Pixi).
+ */
+export function switchableBox({ board, pixi }) {
+  let inner = null, owner;
+  const drop = () => { try { inner?.destroy(); } catch { /* ignore */ } inner = null; owner = undefined; };
+  return {
+    get mesh() { return inner ? inner.mesh || null : null; },
+    get inner() { return inner; },
+    update(cam, b) {
+      const scene = board() || null;
+      if (!inner || owner !== scene || inner.destroyed) {
+        drop();
+        inner = scene ? scene.createDevice() : pixi();
+        owner = scene;
+      }
+      inner?.update(cam, b);
+    },
+    destroy: drop,
+  };
+}