Phase 6f: let = immutable, var = mutable (binding-only)

Bindings now signal mutability the way Rust/Swift do, instead of `let` meaning
"local" and `var` meaning "module-level":

  - `let x = e`  -> immutable binding; a later `x = …` is a compile error
    (`cannot assign to immutable 'x' … use var`).
  - `var x = e`  -> mutable binding, at local OR module scope (position decides
    scope; the keyword decides mutability).
  - `const`      -> unchanged (compile-time).

Immutability is of the *binding*, not the object: `let n = new Node; n.kind = 1`
is fine (mutation through the reference); only rebinding `n` is rejected. The
check lives in emit_assign — a direct `name =` whose target is a `let` local
(loc_mut == 0) errors; field/index targets and `var`/param/loop bindings are
unaffected.

Delivered as three reseeds so the self-hosting compiler never had to compile
source its own rules would reject:
  A) add `var` as a local statement + per-local mutability tracking (loc_mut),
     no enforcement;
  B) migrate every reassigned `let` -> `var` across the compiler, runtime and
     examples (337 declarations), driven by a per-function, string/comment-aware
     scan (binding targets only, never `x.f =` / `x[i] =`);
  C) turn on the check. bootstrap-cfree (compiler vs its own source) and every
     golden build (which splices the runtime) then proved zero reassigned `let`
     was missed anywhere.

Also folded in: removed leftover debug instrumentation in block() (a `cur=` /
print_int(777…) trace on the separator-error path) and fixed parse.ludic's stale
header comment (no more `struct`). Reseeded (21711 lines); C-free fixpoint holds;
goldens identical; 17/17; vocab + doc-fences clean.

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

examples/snake.ludic

@@ -114,14 +114,14 @@ program Snake {
     clear(0x0d1020)
     for gy in 0 .. GH {
       for gx in 0 .. GW {
-        let col = 0x141a2e
+        var col = 0x141a2e
         if (gx + gy) % 2 == 0 { col = 0x10162a }
         fill_rect(gx * TS, gy * TS, TS, TS, col)
       }
     }
     fill_rect(reg(R_FX) * TS + 3, reg(R_FY) * TS + 3, TS - 6, TS - 6, 0xe0405a)
     for (p, s) in query [Pos, Seg] {
-      let col = 0x40c060
+      var col = 0x40c060
       if s.order == 0 { col = 0x9dffb0 }
       fill_rect(p.x * TS + 1, p.y * TS + 1, TS - 2, TS - 2, col)
     }