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-let.md

@@ -5,8 +5,27 @@ category: structure
 kind: keyword
 tokens: let
 sig: let name = value
-tip: An immutable binding, scoped to the block it appears in.
+tip: An immutable binding — the default choice for a value that never changes.
 order: 7
 ---
 
-An immutable binding, scoped to the block it appears in.
+A <code>let</code> introduces an <b>immutable</b> binding: once set, reassigning it (`name = …`) is a compile error. Reach for `let` by default and only switch to `var` when a value genuinely needs to change — it makes intent obvious and catches accidental writes. Immutability is of the <b>binding</b>, not the object it points at: a `let` that holds a property record or a slice still lets you mutate through it (`node.current = 5`), it just cannot be repointed at a different object. Bindings inside a body are locals scoped to their block.
+
+```ludic
+program Damage {
+  property Health { current: int = 100, maximum: int = 100 }
+  model Enemy { Health }
+
+  handler Boot phase Start {
+    spawn Grunt { Health { current: 40 } }
+  }
+
+  handler ApplyHit phase Update {
+    let incoming_damage = 12
+    for (health) in query [Health, {Enemy}] {
+      let survivor = health.current - incoming_damage
+      health.current = survivor
+    }
+  }
+}
+```