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.

tests/expected/190-tuple-values.txt

@@ -9,3 +9,4 @@ concat 1 4
 rep 1 2
 mem true
 lex true
+mixed 42 hi

tests/expected/196-pack-value-projection.exit

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

tests/expected/196-pack-value-projection.txt

@@ -0,0 +1,2 @@
+0=42 1=hi
+0=x 1=7