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-getenv.md

@@ -5,8 +5,24 @@ category: builtins
 kind: builtin
 tokens: getenv
 sig: getenv(name) -> str
-tip: Read an environment variable.
+tip: Read an environment variable, returning empty when it is unset.
 order: 23
 ---
 
-Return the value of environment variable <code>name</code> (empty when it is unset).
+Returns the value of the named environment variable, or an empty string when that variable is not set. Use it to make command-line tools and headless runs configurable without recompiling — reading a data directory, a log level, or a seed from the environment. Check for the empty string to detect an unset variable and fall back to a default. Like the other system builtins, it is aimed at tooling rather than a shipped game loop.
+
+Parameters:
+- `name` — the environment variable name to read
+
+```ludic
+program ReadConfig {
+  handler ShowConfig phase Start {
+    let level = getenv("LUDIC_LEVEL")
+    if len(level) == 0 {
+      print("default")
+    } else {
+      print(level)
+    }
+  }
+}
+```