Document the H5P findings

docs/API.md gains the endpoint and what it returns, why `play` is the bigger
and lesser of the two, the element-by-id route found while probing, and the
per-library shapes with the traps in them — the string "true", the inline cloze
markers, the correct-option-first convention, and the UI subtrees that drown an
exercise in button labels.

CLAUDE.md's "no quiz of its own" note said a contentId was the only handle onto
the content. That was the reason nobody looked further, so it now says where to
look instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

local-instance/README.md

@@ -217,7 +217,7 @@ be pointed at the instance in between:
 
 ```bash
 eval "$(./scripts/mcp-env.sh)"      # as the demo student
-cd .. && npm run smoke              # 80 checks against the local instance
+cd .. && npm run smoke              # 82 checks against the local instance
 ```
 
 `mcp-env.sh` points the index at its own database, `schulcloud_local`, and the