Epic: Phase 2 Multi-Example Loader Implementation

[keepithandy]

Phase

Phase 2 — Extensibility

Goal

Build the first real multi-example loading path on top of the registry and metadata work from v0.4.

This is the issue where Depth Engine starts becoming more than Rat Cellar plus docs, while still preserving the direct index.html starter experience.

Why this matters

The engine needs to prove that Rat Cellar is just one example, not the hidden identity of the project. A loader gives the repo a clean path toward starter templates, example packs, and future ecosystem work.

Proposed work

• Use examples/examples.manifest.js as the source of registered examples.
• Decide whether v1 loader support should be direct-script-only, dynamic-script-based, hosted-only, or a hybrid.
• Add a tiny example selection design, even if implementation starts with one example.
• Define how an active example is chosen.
• Define how content scripts are loaded in order.
• Preserve direct local-file startup or clearly document any mode that requires a server.
• Add smoke coverage for registry integrity and load order.
• Add at least one fake/synthetic registry fixture in smoke tests.
• Keep Rat Cellar as the default loaded example.

Acceptance checks

• Rat Cellar still opens directly from index.html.
• The registry remains generic and content-owned.
• Loader behavior is documented before or alongside implementation.
• New loader logic does not hardcode Rat Cellar.
• No build step is introduced.
• Smokes fail if the active example is missing from the registry.

Guardrails

• Do not add remote third-party content loading in this issue.
• Do not add a large UI framework.
• Do not remove the direct startup path unless there is a separate explicit decision.
• Do not build plugin hooks here; that belongs in the hooks epic.

Suggested follow-up slices

• Loader mode decision doc.
• Registry integrity smoke.
• Static selector markup prototype.
• Dynamic script loader prototype.
• Rat Cellar default-example contract.