feat(bake-sprite): --pixels-per-meter <N> embeds atlas_meta in sprites.uv.json

Sprite-mode bake gains an additive --pixels-per-meter flag. When set,
the value is written into sprites.uv.json under atlas_meta.
pixels_per_meter. Consumers (lib-core.render path-1 via per-entity
sprite_scale) divide their project canonical px/m by this number to
auto-scale the rendered quad — atlas-native resolution no longer needs
to match the project tile-scale.

Unset (legacy bakes): atlas_meta omitted, byte-equal to v0.3.0 output.

Tests: 53/53 pass; existing buildUvJson test continues to assert the
unset-default shape.

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

bin/atlas-baker.js

@@ -24,6 +24,8 @@ function parseArgs(argv) {
             case '--schema':        opts.schema = next; i++; break;
             case '--mode':          opts.mode = next; i++; break;
             case '--strip-prefix':  opts.stripPrefix = next; i++; break;
+            case '--pixels-per-meter':
+                opts.pixelsPerMeter = parseInt(next, 10); i++; break;
             case '--verbose':       opts.verbose = true; break;
             default:
                 console.error(`unknown arg: ${a}`);