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).

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

@@ -1,5 +1,15 @@
 # 0116 — writes through module consts are not rejected (struct-const write bus-errors at runtime)
 
+> **RESOLVED** (2026-06-11, PLAN-CONST-AGG step 2). The assignment
+> lowering rejects any target chain ROOTED at a constant — a
+> const-flagged global (array consts, #run consts) or a module value
+> const (struct consts incl.) — with `cannot assign through constant
+> 'X'`. 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. Regression test:
+> examples/1162-diagnostics-const-write-rejected.sx (struct field — the
+> crash repro, array element, compound, bare scalar).
+
 **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.