lang: const-aggregate comptime folds (PLAN-CONST-AGG step 3)

An array const's '.len' and 'K[<const idx>]' element reads, and a
struct const's field ('LIT.r'), fold as compile-time integer leaves —
usable in array dimensions and other constants' initializers. All
source-aware (the SELECTED author's elements, folded in the author's
context with the cyclic-definition frame); a const out-of-range index
diagnoses at fold time, never wraps.

- evalConstIntExpr gains the three ctx hooks (lookupConstAggLen /
  lookupConstArrayElem / lookupConstStructField) + an index_expr arm;
  all five ctx implementations extended (stateless tiers fold null).
- Array consts dual-register in module_const_map (value = the literal
  node) so the folders see elements; bare reads still hit the GLOBAL
  arm first, so no double emission.
- Untyped consts whose RHS is a const-aggregate leaf ('L :: K.len',
  'E :: K[1]', 'R :: LIT.r') register in a pass 2b AFTER aggregates,
  gated on the receiver naming a const aggregate — a namespaced member
  ('F :: m.PI_ISH') is never mis-typed by the count placeholder.

Examples: 0179 (folds in dims + const exprs), 1163 (OOB diagnostic).

examples/0179-types-const-aggregate-folds.sx

@@ -0,0 +1,23 @@
+// Const aggregates fold at compile time: an array const's `.len` and
+// `K[<const idx>]` element reads, and a struct const's field (`LIT.r`),
+// are compile-time integer leaves — usable in array dimensions and in
+// other constants' initializer expressions, source-aware like every
+// const fold.
+
+#import "modules/std.sx";
+
+Color :: struct { r, g, b: s64; }
+K : [4]s64 : .[11, 22, 33, 44];
+LIT :: Color.{ r = 5, g = 0, b = 0 };
+
+N :: K[0] + K[3];
+L :: K.len;
+R :: LIT.r;
+
+main :: () {
+    a : [K.len]u8 = ---;
+    b : [K[1]]u8 = ---;
+    c : [LIT.r]u8 = ---;
+    print("N={} L={} R={}\n", N, L, R);
+    print("dims={} {} {}\n", a.len, b.len, c.len);
+}

examples/1163-diagnostics-array-const-index-oob.sx

@@ -0,0 +1,12 @@
+// A compile-time index into an array constant is bounds-checked at fold
+// time — out of range is a diagnostic, never a wrap or a silent
+// runtime read.
+
+#import "modules/std.sx";
+
+K : [4]s64 : .[11, 22, 33, 44];
+
+main :: () {
+    b : [K[9]]u8 = ---;
+    print("{}\n", b.len);
+}

examples/expected/0179-types-const-aggregate-folds.exit

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

examples/expected/0179-types-const-aggregate-folds.stderr

@@ -0,0 +1 @@
+

examples/expected/0179-types-const-aggregate-folds.stdout

@@ -0,0 +1,2 @@
+N=55 L=4 R=5
+dims=4 22 5

examples/expected/1163-diagnostics-array-const-index-oob.exit

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

examples/expected/1163-diagnostics-array-const-index-oob.stderr

@@ -0,0 +1,11 @@
+error: index 9 is out of bounds for constant 'K' (4 elements)
+  --> examples/1163-diagnostics-array-const-index-oob.sx:10:10
+   |
+10 |     b : [K[9]]u8 = ---;
+   |          ^^^^
+
+error: array dimension must be a compile-time integer constant
+  --> examples/1163-diagnostics-array-const-index-oob.sx:10:10
+   |
+10 |     b : [K[9]]u8 = ---;
+   |          ^^^^

examples/expected/1163-diagnostics-array-const-index-oob.stdout

@@ -0,0 +1 @@
+