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-arg.md

@@ -1,12 +1,27 @@
 ---
 id: fn-arg
-name: arg_count / arg
+name: arg
 category: builtins
 kind: builtin
-tokens: arg_count arg
-sig: arg_count() -> int   arg(i) -> str
-tip: The command line; argv[0] is included.
-order: 20
+tokens: arg
+sig: arg(i) -> str
+tip: The i-th command-line argument as a string.
+order: 50
 ---
 
-Read the process command line: <code>arg_count()</code> is the number of arguments (including the program name at index 0) and <code>arg(i)</code> returns the <code>i</code>th as a string.
+Returns the command-line argument at index <code>i</code> as a string. Index <code>0</code> is conventionally the program's own name or path, so the first user-supplied argument is at index <code>1</code>. Use it together with <code>arg_count</code> to write configurable command-line tools in Ludic — reading an input filename, a mode flag, or a numeric option. Guard the index against <code>arg_count()</code> before reading so you never ask for an argument that was not passed.
+
+Parameters:
+- `i` — the argument index (`0` is the program name)
+
+```ludic
+program EchoFirstArg {
+  handler ShowArg phase Start {
+    if arg_count() > 1 {
+      print(arg(1))
+    } else {
+      print("no argument")
+    }
+  }
+}
+```