lang: struct consts migrate to const globals, inline fallback (PLAN-CONST-AGG step 4)

A struct constant whose every field serializes — literals, enum tags,
nested aggregates, and (new) const EXPRESSIONS over named consts /
const-aggregate leaves ('r = K + 1', 'g = LIT.r', 'b = A[1]') — becomes
an immutable global: one storage, reads load/GEP it, '@LIT' is
addressable, dead-global elimination drops unused ones. constExprValue
gained a fold-through tail (evalConstIntExpr/evalConstFloatExpr,
source-aware), which also enables const-expression ELEMENTS in array
consts.

A const with a NON-serializable field (a call, a runtime read) keeps
inline re-lowering, and that per-use evaluation is now the documented
contract for the class (pinned: 'CALL.r' reads 1 then 2, side effects
run per use; '#run' is the evaluate-once tool).

Examples: 0180 (migrated shapes + @ptr + copy independence),
0181 (the inline-fallback contract). m3te (23/23) + game rebuilt green.

examples/expected/0180-types-struct-const-globals.exit

@@ -0,0 +1 @@
+0

examples/expected/0180-types-struct-const-globals.stderr

@@ -0,0 +1 @@
+

examples/expected/0180-types-struct-const-globals.stdout

@@ -0,0 +1,3 @@
+lit=255 expr=11 20 8 ref=255
+via-ptr=255
+copy=9 const=255

examples/expected/0181-types-struct-const-inline-fallback.exit

@@ -0,0 +1 @@
+0

examples/expected/0181-types-struct-const-inline-fallback.stderr

@@ -0,0 +1 @@
+

examples/expected/0181-types-struct-const-inline-fallback.stdout

@@ -0,0 +1,3 @@
+use1=1
+use2=2
+counter=2