plan: aggregate consts (PLAN-CONST-AGG) + file 0116 const-write hole

Array-typed '::' consts as immutable globals (approved design: storage
global + untyped inference + comptime-fold layer + const-write rejection
covering the pre-existing struct-const write crash, issue 0116).

issues/0116-const-write-not-rejected.md

@@ -0,0 +1,33 @@
+# 0116 — writes through module consts are not rejected (struct-const write bus-errors at runtime)
+
+**Symptom.** Assigning through a module-level constant compiles silently.
+For a struct-literal const the store lands in read-only memory and the
+program crashes at runtime.
+
+- **Observed**: compiles; `Bus error` at runtime on the store.
+- **Expected**: compile-time diagnostic — `cannot assign through constant
+  'WHITE'`.
+
+## Reproduction
+
+```sx
+#import "modules/std.sx";
+
+Color :: struct { r, g, b: s64; }
+WHITE :: Color.{ r = 255, g = 255, b = 255 };
+
+main :: () {
+    WHITE.r = 0;     // compiles; bus error at runtime
+    print("{}\n", WHITE.r);
+}
+```
+
+(Copies are fine and stay fine: `w := WHITE; w.r = 0;` mutates the copy.)
+
+## Fix
+
+Scheduled as **step 2 of `current/PLAN-CONST-AGG.md`** (aggregate consts):
+one rejection rule for assignment/compound-assignment targets whose ROOT
+identifier resolves to a module const (module_const_map author) or a
+const-flagged global (the array consts that plan introduces). Diagnose at
+the assignment site; pin the repro above as a diagnostics example.