Add lock-file load and stable ID assignment

Lock-file determinism guarantees re-bakes do not renumber existing
tiles. Atlas_id mismatch is a hard-fail to prevent silent corruption.
Deleted tile IDs move to deleted[] and are not recycled.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lock.js

@@ -0,0 +1,69 @@
+// src/lock.js
+// Load + update + serialize the tiles.atlas.lock.json file.
+
+const fs = require('node:fs');
+
+function loadLock(lockPath) {
+    if (!fs.existsSync(lockPath)) {
+        return null;  // signal: fresh bake
+    }
+    const raw = JSON.parse(fs.readFileSync(lockPath, 'utf8'));
+    if (typeof raw.atlas_id !== 'string') {
+        throw new Error(`lock: missing atlas_id in ${lockPath}`);
+    }
+    return {
+        atlas_id: raw.atlas_id,
+        bindings: raw.bindings || {},
+        deleted: raw.deleted || [],
+        next_id: raw.next_id || 1,
+    };
+}
+
+function assignIds(sources, existingLock, atlasId) {
+    if (existingLock && existingLock.atlas_id !== atlasId) {
+        throw new Error(
+            `lock: atlas_id mismatch (lock has '${existingLock.atlas_id}', `
+            + `cli passed '${atlasId}'); refusing to bake`
+        );
+    }
+    const bindings = existingLock ? { ...existingLock.bindings } : {};
+    const deletedSet = new Set(existingLock ? existingLock.deleted : []);
+    let nextId = existingLock ? existingLock.next_id : 1;
+
+    const presentNames = new Set(sources.map(s => s.name));
+    const tiles = [];
+
+    // Assign IDs to all sources (existing → stable, new → next_id++)
+    for (const source of sources) {
+        let id = bindings[source.name];
+        if (id === undefined) {
+            id = nextId++;
+            bindings[source.name] = id;
+        }
+        tiles.push({ ...source, id });
+    }
+
+    // Detect deletions: previously-bound names not in sources
+    for (const name of Object.keys(bindings)) {
+        if (!presentNames.has(name)) {
+            deletedSet.add(bindings[name]);
+            delete bindings[name];
+        }
+    }
+
+    return {
+        tiles,
+        lock: {
+            atlas_id: atlasId,
+            bindings,
+            deleted: [...deletedSet].sort((a, b) => a - b),
+            next_id: nextId,
+        },
+    };
+}
+
+function serializeLock(lock) {
+    return JSON.stringify(lock, null, 2);
+}
+
+module.exports = { loadLock, assignIds, serializeLock };