feat(hud): ship free default stat icons (visible in Studio Edit)

Owner relaxed the asset side of the content-free rule — assets in this repo are
free to use — so the engine now ships its generated HUD icons as defaults instead
of leaving blank slots. Two wins: the HUD is iconed out of the box, and the icons
render in Studio's EDIT view (no Play needed), so you can author/tune the HUD and
see the real thing — or edit it straight from the Explorer.

- StatDefs.luau: each of the 7 stats carries an `icon` default. Putting it here
  (not as a per-bar HUD attribute) keeps the admin-panel/config override working —
  a per-bar attribute would shadow it. resolveIcon: per-bar attr > config/def.icon
  (now the shipped id) > Assets > "".
- assets/hud/SurvivalHud.model.json: bake each bar's child `Icon` ImageLabel
  (Image + Visible=true) so it shows in edit mode; the Credits counter (no StatDefs
  entry) also gets a per-bar `Icon` attribute to drive its runtime resolution.
- Retire demo/client/HudIcons.client.luau (+ its demo.project.json mount): the
  runtime icon-assigning script is now redundant.
- Relax the docs/PR-template "content-free" wording to "design-free": free default
  ART may ship (overridable per stat); game-specific DESIGN (items/recipes/lore)
  still never ships. Updated design-language, survival-stats, CONTRIBUTING.
- CHANGELOG: record default icons + add the missing admin-plugin entry; note CI
  now builds all three Rojo targets.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/pull_request_template.md

@@ -13,6 +13,6 @@
 ## Checklist
 
 - [ ] Added an entry to **`CHANGELOG.md`** under `## Unreleased` (or applied the `skip-changelog` label if no entry is warranted — e.g. a CI-only or trivial docs change)
-- [ ] CI is green — `stylua --check`, `selene`, `luau-lsp analyze`, and **both** `rojo build`s pass
+- [ ] CI is green — `stylua --check`, `selene`, `luau-lsp analyze`, and **all three** `rojo build`s (engine, demo, plugin) pass
 - [ ] No secrets or personal data added (this is a public repo)
-- [ ] **No hardcoded asset IDs** — content comes via `register()` / components / `Assets`, and the core stays content-free
+- [ ] **No game-specific design content** — items / recipes / lore / world strings enter via `register()` / components / `Assets`, not the engine (free *default* art, e.g. HUD icons, is fine)