docs(site): vertical pipeline on mobile, click tooltips, consistent chrome

- Pipeline diagram stacks vertically with downward arrows on mobile (row on
  >=720px) instead of the awkward wrap.
- Tooltip cards now open on CLICK (works on touch too), the card itself is
  clickable and opens the symbol's page in a NEW TAB, and an outside click or
  Escape closes it. Replaces the hover-only behavior.
- Reference pages get their own nav — Home / API Reference / Source ↗ — instead
  of the landing-only Features/Examples/Get started anchors, and now include the
  Source link.
- Unify the container width (1120px) across the landing and all reference pages
  so the header and content align between them.
- Reword extern/@export FFI docs from "C-ABI" to "native" for consistency with
  the site's no-C wording.

Verified in-browser at 375px and 1280px: vertical pipeline, click→card→new-tab,
outside-click close, correct reference nav, aligned container; 158 examples still
compile.

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

docs/language/structure/kw-extern.md

@@ -5,11 +5,11 @@ category: structure
 kind: keyword
 tokens: extern
 sig: extern fn name(a: T) -> R = "symbol"
-tip: Bind a name to an external C-ABI symbol — the seam for platform and library calls.
+tip: Bind a name to an external native symbol — the seam for platform and library calls.
 order: 12
 ---
 
-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`, …).
+An <code>extern fn</code> declares a function whose body lives outside Ludic and binds it to a native symbol resolved at link time. It is the seam through which Ludic reaches anything with a native 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