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>

LANGUAGE.md

@@ -466,11 +466,37 @@ another `.ludic` file (see `examples/lib/`).
 
 ## Statements
 
-`let x = expr` · `x = expr` (`+= -= *= /=`) · `if cond { }` / `if/else`
-(the `else` is optional) · `while cond { }` · `for i in a .. b { }` (numeric range) ·
-`for (…) in query […] { }` · `break` · `continue` · `return` · `spawn` ·
-`despawn` · `enable` / `disable` (a property `on e`, a model, or a handler) ·
-`match` · `machine`.
+`let x = expr` / `var x = expr` · `x = expr` (`+= -= *= /=`) · `if cond { }` /
+`if/else` (the `else` is optional) · `while cond { }` · `for i in a .. b { }`
+(numeric range) · `for (…) in query […] { }` · `break` · `continue` · `return` ·
+`spawn` · `despawn` · `enable` / `disable` (a property `on e`, a model, or a
+handler) · `match` · `machine`.
+
+### Bindings: `let`, `var`, `const`
+
+A binding's keyword states whether it can be reassigned, the way Rust and Swift
+use them — not its scope (position decides that: inside a body it is a local,
+at the top level it is module state).
+
+- **`let x = e`** — an *immutable* binding. `x = …` afterward is a compile error
+  (`cannot assign to immutable 'x'`). Reach for `let` by default.
+- **`var x = e`** — a *mutable* binding: `x`, `x += 1`, … reassign it. Use it for
+  loop accumulators and anything that genuinely changes.
+- **`const NAME = e`** — a compile-time constant (folded, no storage).
+
+Immutability is of the **binding**, not the object. A `let` that holds a record
+or slice still lets you mutate *through* it — the reference itself just cannot be
+repointed:
+
+```ludic
+# doc-check: skip — illustrative bindings
+let n = new Node       # immutable binding…
+n.kind = 1             # …but mutation through it is fine
+n = new Node           # ERROR: cannot assign to immutable 'n'
+
+var total = 0
+for i in 0 .. 10 { total += i }   # a var is the right tool for an accumulator
+```
 
 **Statements are separated by a newline or `;`** (both lex to the same separator
 token). Two statements may not sit adjacent with only spaces between them — the