ir: fix tuple literal element widths (construction was garbage)

A tuple_init's element values must match its field types exactly — LLVM
`insertvalue` does no implicit conversion. An inferred `pair := (40, 2)`
lowered its elements under the enclosing fn's `target_type` (e.g. main's
s32 return), producing i32 values, while the field types were inferred
independently as s64. The {i64,i64} aggregate was filled with i32
constants, so reading any element back returned garbage (40 + 2^32) and
tuple equality was always false.

lowerTupleLiteral now lowers each element under its resolved field type
(the contextual target tuple's fields when present, else per-element
inference) and coerces to it, so value width always matches field width.
Assignment to a tuple-typed field/element now also propagates the target
tuple type. Adds examples/190-tuple-values.sx as a regression test and
examples/probes/tuple-baseline.sx as the Step 0.4 audit artifact.

examples/190-tuple-values.sx

@@ -0,0 +1,49 @@
+// Tuple values: construction, element access, struct-field storage,
+// return, and operators. Regression for the tuple-construction bug where
+// an inferred `:=` tuple literal lowered its element values under the
+// enclosing fn's (narrower) return `target_type`, mismatching the
+// independently-inferred s64 field types and yielding garbage on read.
+
+#import "modules/std.sx";
+
+Box :: struct { xs: (s32, s32); }
+
+swap :: (a: s64, b: s64) -> (s64, s64) { (b, a); }
+fst  :: (t: (s64, s64)) -> s64 { t.0; }
+
+main :: () -> s32 {
+    // Inferred positional tuple + numeric field access.
+    pair := (40, 2);
+    print("pair {} {}\n", pair.0, pair.1);
+
+    // Named tuple: named + numeric access.
+    named := (x: 10, y: 20);
+    print("named {} {} {}\n", named.x, named.0, named.1);
+
+    // Element into a typed local (access path, not just print).
+    a : s64 = pair.0;
+    b : s64 = pair.1;
+    print("locals {} {}\n", a, b);
+
+    // Tuple-typed struct field: store a tuple value, read both elements.
+    box : Box = ---;
+    box.xs = (7, 9);
+    print("field {} {}\n", box.xs.0, box.xs.1);
+
+    // Return a tuple from a function.
+    s := swap(1, 2);
+    print("ret {} {}\n", s.0, s.1);
+
+    // Pass a tuple by value.
+    print("pass {}\n", fst((11, 22)));
+
+    // Operators: equality, concatenation, repetition, membership, lex.
+    print("eq {}\n", (1, 2) == (1, 2));
+    c := (1, 2) + (3, 4);
+    print("concat {} {}\n", c.0, c.3);
+    r := (1, 2) * 3;
+    print("rep {} {}\n", r.0, r.5);
+    print("mem {}\n", 3 in (1, 2, 3));
+    print("lex {}\n", (1, 2) < (1, 3));
+    0;
+}