Support files can now either embed inline string content (Inline) or
reference a file path on disk to copy (Copy_from). This enables large
binary-like files such as x-ocaml.js (~16MB) to be managed via
`odoc support-files` without embedding them as OCaml string literals.

The scrollycode extension's x-ocaml script URLs are now configurable
via ODOC_X_OCAML_JS and ODOC_X_OCAML_WORKER environment variables,
and ODOC_X_OCAML_JS_PATH registers the file as a Copy_from support file.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Three-layer architecture separating content, theme, and renderer:

Phase 1 — Scrollycode refactor:
- Strip ~1030 lines of embedded theme CSS from scrollycode_extension.ml
- Create scrollycode_css.ml with structural CSS using CSS custom properties
- Create scrollycode_themes.ml with three theme files (warm/dark/notebook)
registered as support files
- Simplify @scrolly tag: theme suffix now ignored (CSS concern, not content)
- Playground overlay styled via --xo-* custom properties on x-ocaml element

Phase 2 — Extra CSS support:
- Add --extra-css flag to odoc html-generate for injecting additional
<link> tags (used for per-page theme selection)

Phase 3 — Shell plugin system:
- Add Html_shell module with Shell interface and hashtable registry
- Register "default" shell in html_page.ml, "json" shell in
html_fragment_json.ml
- Replace hardcoded if/else in generator.ml with shell registry lookup
- Add --shell NAME flag (--as-json kept as backward-compat alias)

x-ocaml theming:
- Replace hardcoded colors in style.css with var(--xo-*, fallback)
covering editor, gutter, buttons, tooltips, and output areas
- CSS custom properties inherit through shadow DOM boundary, so
consumers theme x-ocaml with pure CSS — no JS injection needed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>