Add optional GLB compression stage

.trellis/tasks/08-03-experiment-glb-compression/prd.md

@@ -0,0 +1,55 @@
+# Experiment GLB compression
+
+## Goal
+
+Create and validate a repeatable GLB compression experiment for
+`nantaizi-lake-innovation-valley`, focused on reducing file size without making
+the current uncompressed GLB output the wrong baseline.
+
+Current measurement:
+
+- `nantaizi-lake-innovation-valley.glb`: `20.50 MB`
+- embedded images: `17.72 MB`
+- non-image binary buffer: `2.45 MB`
+- conclusion: texture compression / resize should be tested before geometry
+  compression.
+
+## Requirements
+
+1. Add a standalone compression tool or script that consumes an existing GLB and
+   writes a separate compressed GLB output.
+2. Do not replace the default `<area>.glb` in this first experiment.
+3. Prefer texture-first compression; geometry compression may be evaluated but
+   should not be treated as the primary win.
+4. Keep the experiment repeatable from the command line.
+5. Validate only `nantaizi-lake-innovation-valley`.
+6. After visual review accepts the WebP result, integrate it as an explicit
+   optional `compress` stage that does not run in `all` and does not replace the
+   default GLB.
+7. Report:
+   - source GLB size
+   - compressed GLB size
+   - image vs non-image size before compression
+   - GLB extensions introduced
+   - Cesium preview compatibility status or blocker
+
+## Acceptance Criteria
+
+- [x] Compression command/script runs from this repo.
+- [x] It produces a separate compressed GLB for nantaizi.
+- [x] Source GLB is left intact.
+- [x] `scripts/glb-digest.js` can inspect the compressed GLB or the blocker is
+      documented if the digest needs extension support.
+- [x] Visual/Cesium compatibility next step is clear.
+- [x] Optional `compress` stage can generate compressed nantaizi artifacts from
+      the existing Cesium outputs.
+- [x] `compress` is not included in `all` and cannot be enabled from area
+      `stages`.
+- [x] No `outputs/` files are committed.
+
+## Out Of Scope
+
+- Making compressed GLB replace the default pipeline artifact.
+- hanyang validation.
+- CDN/server gzip/brotli configuration.
+- Large frontend UI changes.