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/structure/kw-extern.md

@@ -9,4 +9,23 @@ tip: Bind a name to an external C-ABI symbol — the seam for platform and libra
 order: 12
 ---
 
-Bind a name to an external C-ABI symbol — the seam for platform and library calls.
+An <code>extern fn</code> declares a function whose body lives outside Ludic and binds it to a C-ABI symbol resolved at link time. It is the seam through which Ludic reaches anything with a C interface — a system library, a math routine, or even another `.ludic` file compiled as a `module`. You write the Ludic signature you want to call and give the real symbol name after `=`; the linker connects them (pass `-L`/`-l` to `ludicc` to point at the library). Types must match the foreign ABI, so map each parameter and the return to the right Ludic type (`int`, `fixed`, `ptr`, …).
+
+Parameters:
+- `a` — a typed argument passed straight through to the foreign symbol
+
+```ludic
+program Physics {
+  extern fn c_hypot(a: fixed, b: fixed) -> fixed = "hypot_fx"
+
+  property Velocity { delta_x: int = 0, delta_y: int = 0 }
+  model Projectile { Velocity }
+
+  handler MeasureSpeed phase Update {
+    for (velocity) in query [Velocity, {Projectile}] {
+      let speed = c_hypot(a: fx(velocity.delta_x), b: fx(velocity.delta_y))
+      Screen.status(str(flr(speed)))
+    }
+  }
+}
+```