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;
 }