lang: reject writes through constants (PLAN-CONST-AGG step 2, fixes 0116)

Any assignment / compound-assignment whose target chain is ROOTED at a
constant — a const-flagged global (array consts, #run consts) or a
module value const (struct consts incl.) — diagnoses 'cannot assign
through constant X' at compile time. A struct const's field write used
to compile and bus-error at runtime (issue 0116); scalars misfired
silently. A deref along the chain (p.*) breaks the root — pointer
writes stay the documented escape until the const-ness steps; a local
shadowing the const name stays writable.

Also: typed struct constants ('W : Color : Color.{...}') register —
the shape list skipped struct_literal, leaving the typed form
unresolved while the untyped one worked.

Examples: 1162 (all rejection shapes incl. the 0116 crash repro),
0178 (typed struct const reads + copy independence).

examples/1162-diagnostics-const-write-rejected.sx

@@ -0,0 +1,19 @@
+// Writes through a module constant are compile errors — for every target
+// chain rooted at a const: a struct const's field (this used to compile
+// and BUS-ERROR at runtime — issue 0116), an array const's element,
+// a compound assignment, and a bare scalar const. A local that shadows
+// the const name stays writable (see 0177/0178 for the value semantics).
+
+#import "modules/std.sx";
+
+Color :: struct { r, g, b: s64; }
+WHITE :: Color.{ r = 255, g = 255, b = 255 };
+K : [4]s64 : .[11, 22, 33, 44];
+N : s64 : 4;
+
+main :: () {
+    WHITE.r = 0;
+    K[0] = 5;
+    K[1] += 2;
+    N = 9;
+}