docs: const-aggregate semantics + unchecked-pointer contract (PLAN-CONST-AGG step 6)

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

@@ -5,7 +5,8 @@
 > 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
+> are permanently unchecked — the documented pointer contract, specs.md
+> §Pointer Types); 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).