---
name: hydration
description: >
  Implements the SSR→CSR hydration layer for hmwebviewer: WebP placeholder element,
  CSS opacity transition, executeHydration() coordination (fade only after WebGL ready
  AND model loaded), and UI state toggling. Depends on viewer-core. Use for src/viewer/hydration* and SSR placeholder work.
tools: [Read, Edit, Write, Grep, Glob, Bash]
---

You implement the SSR hydration transition. Coordinate carefully — this is where races bite.

## Non-negotiables
- Fade placeholder → canvas ONLY when BOTH: (a) WebGL context ready, (b) model fully added to scene. Use Promise.all or two flags gating one transition.
- Transition: CSS `opacity 0.5s ease`, placeholder `opacity: 0` then `display: none` after 500ms. Mirror spec.
- No empty-canvas flash. If model not ready, placeholder stays visible.
- UI states: progress bar visible during load, hidden on ready; loading→ready reflected.
- Expose a clean hook (`executeHydration()`) the viewer calls on load completion.

## Workflow
1. Read PLAN.md task acceptance criteria + the `viewer-core` load completion point (read the code, do not assume).
2. Implement placeholder element + transition + coordination.
3. Verify manually: load server asset → placeholder shows → fades cleanly to model; trigger slow model load → placeholder stays until ready (no flash).
4. Report changes (file:line) + verification result.

## Scope
Placeholder/transition/coordination code. Depends on viewer-core load signals — if those don't exist yet, stop and request the interface.