docs(api): per-symbol pages, fuzzy search, deep token linking, hover cards

Rebuild the API Reference around one page per symbol and richer, verified content.

Pages & navigation
- One HTML page per symbol (kw-*, type-*, phase-*, screen-*, fn-*, annot-*, op-*)
  instead of a single scrolling page; namespace overview pages (ns-screen …
  ns-color) and a searchable index (api.html) with client-side fuzzy search.
- Sticky-header scroll offset (scroll-margin) so a jumped-to entry/param/color is
  never hidden, plus a flash highlight on the scrolled-to target.

Deep linking in every snippet & example
- Namespace members split: `Screen`→namespace page, `fill_rectangle`→method page;
  `Color`→palette page, `Charcoal`→its swatch — separately.
- Named arguments (`width:`) link to that parameter's anchor on the method page.
- Hover any token for a summary card built from the real API data (symbols.json).

Content & coverage
- Full authoritative surface documented from the compiler: every keyword, type,
  the 6 phases (Start/Input/FixedUpdate/Update/LateUpdate/Render, each its own
  page), all 22 annotations, namespace methods with parameter docs, builtins,
  the world_* reflection ABI, networking, operators — 155 symbols.
- Longer, clearer explanations; "model"/"model instance" terminology, not "entity";
  descriptive identifiers in every example (Position{column,row}, Velocity{delta_x,
  delta_y}, Health{current,maximum}, Player/Enemy) — no Pos/Seg/x/dx.
- Accuracy fixes from compiler ground-truth: world_count() takes no arg,
  world_query_next(property, cursor) arg order, event fields bind by name; dropped
  `when` and `module` (not in the self-hosted parser).

Tooling
- inventory.json + check.py: coverage guard (every symbol has a page), duplicate-
  token guard, and broken-link guard — fail CI so docs can't drift.
- validate.py: compiles every ```ludic example against bin/ludicc (158 compile).

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

docs/language/builtins/fn-str.md

@@ -5,8 +5,22 @@ category: builtins
 kind: builtin
 tokens: str
 sig: str(x) -> str
-tip: Convert an int/bool/fixed to text; a string passes through.
+tip: Convert an int, bool, or fixed value to text; a string passes through unchanged.
 order: 1
 ---
 
-Convert an int/bool/fixed to text; a string passes through. Also used by <code>`{…}`</code> interpolation.
+Converts a value to its textual form: an <code>int</code>, <code>bool</code>, or <code>fixed</code> becomes a string, and a value that is already a string is returned unchanged. This is what backtick <code>`{…}`</code> interpolation calls under the hood, so most of the time you can interpolate directly instead of calling <code>str</code> yourself. Reach for the explicit form when you need to store or pass around the text, or build a string in pieces. The result can be printed, drawn with <code>Screen.draw_text</code>, or concatenated.
+
+Parameters:
+- `x` — the value to convert to text
+
+```ludic
+program LabelValue {
+  var score: int = 250
+
+  handler ReportScore phase Start {
+    let label = str(score)
+    print(label)
+  }
+}
+```