Fix undo/redo forward-state restore, harden test infra, skip-guard

Root cause: mutation writers stored oldValues only. Redo rebuilt from
current state via derived logic — wrong order, wrong content, or no-op.
Undo patched fields in place but ignored roster/array order changes.

Writers now store forward arrays (shared/turn.js):
- addParticipant: newTurnOrderIds, newCurrentTurnParticipantId
- removeParticipant: newTurnOrderIds, newCurrentTurnParticipantId
- updateParticipant: oldTurnOrderIds + newTurnOrderIds (initiative re-slot)
- reorderParticipants: newParticipants + newTurnOrderIds
- damage/heal/deathSave/stabilize/revive/deactivate_dead_monster:
  oldValues + newValues both capture full death state incl conditions

expandUndo redo uses stored forward state instead of deriving:
- add/remove: order via newTurnOrderIds
- update: initiative change re-orders both directions
- reorder: re-applies newParticipants
- death ops: restore via newValues (was HP-only, dropped status/conditions)

Missing expandUndo cases added: stabilize, revive, deactivate_dead_monster
(were default:null -> undo no-op).

Test infra (shared/tests/_helpers.js):
- Mock storage undo() real now: applies patch + flips undone flag (was no-op)
- addDoc persists log entries, getCollection returns them
- undoLast/redoLast harness helpers exercise real mechanism
- Undo tests assert against actual persisted doc, not expandUndo output

turn.undo.test.js rewritten: every op tested as undo->deepEqual before,
redo->deepEqual after-op (12 cases). Was undo-only, redo untested.
turn.deathsave.undo.test.js added: 11 death-path roundtrips.

Dead code removed:
- round-trip.test.js: skipped suite testing deleted replay-from-logs.js
  (5 skipped tests rotting). Coverage gap acknowledged in TODO.

Skip-guard added (scripts/run-tests.sh): pre-flight grep refuses to run
if any .skip/xdescribe/xit found in test dirs. No CI; this is the gate.

shared/tests/_helpers.js

@@ -1,9 +1,18 @@
 // Test helper: in-memory mock storage. Records all writes (addDoc/updateDoc).
-// Captures encounter patches + log entries so tests assert persistence.
+// Stores docs so getDoc/getCollection return real state. undo() actually
+// applies patches + flips log `undone` flag (mirrors firebase/server contract).
 function createMockStorage() {
   const docs = new Map();        // path -> data
   const calls = [];              // ordered {fn, path, data}
 
+  const applyPatch = (path, patch) => {
+    if (!docs.has(path)) docs.set(path, {});
+    const cur = docs.get(path);
+    // Firestore shallow-merge: arrays replaced, not merged. Patch arrays are
+    // the FULL new value (participants, turnOrderIds, conditions).
+    docs.set(path, { ...cur, ...patch });
+  };
+
   return {
     calls,
     docs,
@@ -14,24 +23,66 @@ function createMockStorage() {
     },
     async addDoc(path, data) {
       calls.push({ fn: 'addDoc', path, data });
+      // Persist log entry so getCollection + undo can read it back.
+      const id = data.id || `log-${calls.length}`;
+      const stored = { ...data, id, _path: `${path}/${id}` };
+      docs.set(`${path}/${id}`, stored);
+      return id;
     },
     async updateDoc(path, patch) {
       calls.push({ fn: 'updateDoc', path, data: patch });
-      if (!docs.has(path)) docs.set(path, {});
-      docs.set(path, { ...docs.get(path), ...patch });
+      applyPatch(path, patch);
     },
     async deleteDoc(path) {
       calls.push({ fn: 'deleteDoc', path });
       docs.delete(path);
     },
-    async getCollection() { return []; },
+    async getCollection(path, queryConstraints = []) {
+      // Return docs whose _path starts with `${path}/` (Firestore collection).
+      const out = [];
+      for (const [p, d] of docs.entries()) {
+        if (p.startsWith(`${path}/`)) out.push({ ...d });
+      }
+      // Best-effort query support: where/orderBy/limit.
+      let filtered = out;
+      for (const qc of queryConstraints) {
+        if (!filtered.length) break;
+        const op = qc[0];
+        if (op === 'where') {
+          const [, field, , value] = qc;
+          filtered = filtered.filter(d => d[field] === value);
+        } else if (op === 'orderBy') {
+          const [, field, dir = 'asc'] = qc;
+          filtered.sort((a, b) => {
+            const av = a[field], bv = b[field];
+            if (av < bv) return dir === 'asc' ? -1 : 1;
+            if (av > bv) return dir === 'asc' ? 1 : -1;
+            return 0;
+          });
+        } else if (op === 'limit') {
+          filtered = filtered.slice(0, qc[1]);
+        }
+      }
+      return filtered;
+    },
     async batchWrite(ops) {
       for (const op of ops) {
         if (op.type === 'delete') await this.deleteDoc(op.path);
         else await this.updateDoc(op.path, op.data);
       }
     },
-    async undo() {},
+    // Transactional undo — mirrors firebase.js / server.js contract:
+    //   redo=false: apply undo.updates to encounterPath, flip log undone=true
+    //   redo=true:  apply undo.redo to encounterPath, flip log undone=false
+    async undo({ logPath, undo, redo = false }) {
+      calls.push({ fn: 'undo', path: logPath, data: { undo, redo } });
+      const patch = redo ? (undo.redo || undo.updates) : undo.updates;
+      applyPatch(undo.encounterPath, patch);
+      if (docs.has(logPath)) {
+        const log = docs.get(logPath);
+        docs.set(logPath, { ...log, undone: !redo });
+      }
+    },
     // filters
     updatesFor(prefix) { return calls.filter(c => c.fn === 'updateDoc' && c.path.startsWith(prefix)); },
     logs() { return calls.filter(c => c.fn === 'addDoc').map(c => c.data); },
@@ -39,6 +90,31 @@ function createMockStorage() {
   };
 }
 
+// Undo/redo harness: apply op via shared func (writes enc doc + log) then
+// exercise the REAL undo mechanism (storage.undo applies patch + flips flag).
+// Reads doc back so assertions reflect actual persisted state.
+//   applyThenUndo(ctx, encAfterOp) -> enc after undo applied
+//   applyThenRedo(ctx, encAfterUndo, lastLogId) -> enc after redo applied
+async function undoLast(ctx, encAfterOp) {
+  const { storage, encPath, logPath } = ctx;
+  const last = storage.logs()[storage.logs().length - 1];
+  const { expandUndo } = require('./..');
+  const expanded = expandUndo(last, encAfterOp);
+  if (!expanded) throw new Error('expandUndo returned null');
+  const logEntryPath = `${logPath}/${last.id}`;
+  await storage.undo({ logPath: logEntryPath, undo: expanded, redo: false });
+  return await storage.getDoc(encPath);
+}
+async function redoLast(ctx, encAfterUndo, last) {
+  const { storage, encPath, logPath } = ctx;
+  const { expandUndo } = require('./..');
+  const expanded = expandUndo(last, encAfterUndo);
+  if (!expanded) throw new Error('expandUndo returned null (redo)');
+  const logEntryPath = `${logPath}/${last.id}`;
+  await storage.undo({ logPath: logEntryPath, undo: expanded, redo: true });
+  return await storage.getDoc(encPath);
+}
+
 // Standard ctx for tests. encPath + logPath + displayPath preset.
 // Returns { storage, ctx } so tests destructure both in one call:
 //   const { storage, ctx } = mockCtx();
@@ -70,4 +146,4 @@ function readyEnc(opts = {}) {
   };
 }
 
-module.exports = { createMockStorage, mockCtx, readyEnc };
+module.exports = { createMockStorage, mockCtx, readyEnc, undoLast, redoLast };