lang 2.4: value-position pack projection xs.value + mixed-tuple type fix

`xs.<method>` over a constrained pack projects a (zero-arg) protocol method
across every element into a tuple: `xs.get` ≈ `(xs[0].get(), …, xs[N-1].get())`.
lowerFieldAccess intercepts `xs.<m>` on a pack base (where <m> is a protocol
method) and synthesizes/lowers `xs[i].<m>()` per element into a tuple_init.
For a parameterised `Box(T)` the projected tuple is heterogeneous (each element
returns its own T). examples/196-pack-value-projection.sx.

Surfaced and fixed a pre-existing bug: inferExprType didn't handle tuple field
access (`t.0` / `t.x`), so a mixed-size tuple like `(42, "hi")` inferred the
string field as s64 — the wrong type then drove a bad `print` pack mangle and
coerced the string to i64 (garbage). Added the tuple arm (numeric + named).
Regression: a `(s64, string)` case in examples/190-tuple-values.sx.

examples/190-tuple-values.sx

@@ -45,5 +45,11 @@ main :: () -> s32 {
     print("rep {} {}\n", r.0, r.5);
     print("mem {}\n", 3 in (1, 2, 3));
     print("lex {}\n", (1, 2) < (1, 3));
+
+    // Mixed-size fields: a tuple with both an s64 and a string (16-byte fat
+    // pointer). Field types are tracked per-position, so reading each back is
+    // typed correctly (s64 prints as a number, string as text).
+    mixed := (42, "hi");
+    print("mixed {} {}\n", mixed.0, mixed.1);
     0;
 }

examples/196-pack-value-projection.sx

@@ -0,0 +1,28 @@
+// Feature 1 — value-position pack projection: `xs.<method>` projects a
+// (zero-arg) protocol method over every element into a TUPLE of the per-element
+// results. For a parameterised `Box(T)`, each element's method returns its own
+// `T`, so the projected tuple is heterogeneous.
+//
+//   xs.get  ≈  (xs[0].get(), xs[1].get())
+
+#import "modules/std.sx";
+
+Box :: protocol(T: Type) {
+    get :: () -> T;
+}
+
+IntCell :: struct { v: s64; }
+StrCell :: struct { s: string; }
+impl Box(s64) for IntCell { get :: (self: *IntCell) -> s64 => self.v; }
+impl Box(string) for StrCell { get :: (self: *StrCell) -> string => self.s; }
+
+show :: (..xs: Box) -> void {
+    vals := xs.get;                       // tuple (s64, string)
+    print("0={} 1={}\n", vals.0, vals.1);
+}
+
+main :: () -> s32 {
+    show(IntCell.{ v = 42 }, StrCell.{ s = "hi" });
+    show(StrCell.{ s = "x" }, IntCell.{ v = 7 });   // order swapped → (string, s64)
+    0;
+}