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;
+}

src/ir/lower.zig

@@ -3976,6 +3976,19 @@ pub const Lowering = struct {
             }
         }
 
+        // Pack value projection: `xs.<m>` where `<m>` is a (zero-arg) method of
+        // the pack's constraint protocol projects it over every element →
+        // a tuple `(xs[0].<m>(), …, xs[N-1].<m>())`. (`xs.len` handled above.)
+        if (self.pack_constraint) |pcon| {
+            if (fa.object.data == .identifier) {
+                if (pcon.get(fa.object.data.identifier.name)) |proto| {
+                    if (self.lookupProtocolField(proto, fa.field) != null) {
+                        return self.lowerPackValueProjection(fa.object.data.identifier.name, fa.field, span);
+                    }
+                }
+            }
+        }
+
         // Interface-only enforcement (Decision): a member access on a
         // constrained pack element `xs[i].<m>` may only name a method of the
         // constraint protocol — not an arbitrary concrete field. Checked here,
@@ -4078,6 +4091,41 @@ pub const Lowering = struct {
         return self.lowerFieldAccessOnType(obj, obj_ty, fa.field, span);
     }
 
+    /// Value-position pack projection `xs.<method>`: call the (zero-arg)
+    /// protocol method on each element and collect the results into a tuple
+    /// `(xs[0].<method>(), …, xs[N-1].<method>())`. N=0 yields the empty tuple.
+    /// Synthesizes `xs[i].<method>()` per element and lowers it (substitution
+    /// turns `xs[i]` into the concrete arg; UFCS dispatches the method).
+    fn lowerPackValueProjection(self: *Lowering, pack_name: []const u8, method: []const u8, span: ast.Span) Ref {
+        const n: u32 = if (self.pack_param_count) |ppc| (ppc.get(pack_name) orelse 0) else 0;
+        var refs = std.ArrayList(Ref).empty;
+        defer refs.deinit(self.alloc);
+        var tys = std.ArrayList(TypeId).empty;
+        defer tys.deinit(self.alloc);
+        var i: u32 = 0;
+        while (i < n) : (i += 1) {
+            const id_node = self.alloc.create(Node) catch return self.builder.constInt(0, .void);
+            id_node.* = .{ .span = span, .data = .{ .identifier = .{ .name = pack_name } } };
+            const idx_node = self.alloc.create(Node) catch return self.builder.constInt(0, .void);
+            idx_node.* = .{ .span = span, .data = .{ .int_literal = .{ .value = @intCast(i) } } };
+            const index_node = self.alloc.create(Node) catch return self.builder.constInt(0, .void);
+            index_node.* = .{ .span = span, .data = .{ .index_expr = .{ .object = id_node, .index = idx_node } } };
+            const fa_node = self.alloc.create(Node) catch return self.builder.constInt(0, .void);
+            fa_node.* = .{ .span = span, .data = .{ .field_access = .{ .object = index_node, .field = method } } };
+            const call_node = self.alloc.create(Node) catch return self.builder.constInt(0, .void);
+            call_node.* = .{ .span = span, .data = .{ .call = .{ .callee = fa_node, .args = &.{} } } };
+            const r = self.lowerExpr(call_node);
+            refs.append(self.alloc, r) catch return self.builder.constInt(0, .void);
+            tys.append(self.alloc, self.builder.getRefType(r)) catch return self.builder.constInt(0, .void);
+        }
+        const tuple_ty = self.module.types.intern(.{ .tuple = .{
+            .fields = self.alloc.dupe(TypeId, tys.items) catch return self.builder.constInt(0, .void),
+            .names = null,
+        } });
+        const owned = self.alloc.dupe(Ref, refs.items) catch return self.builder.constInt(0, .void);
+        return self.builder.emit(.{ .tuple_init = .{ .fields = owned } }, tuple_ty);
+    }
+
     /// Lower a struct-level constant value (e.g., Phys.GRAVITY).
     fn lowerStructConstant(self: *Lowering, info: StructConstInfo) Ref {
         const val_node = info.value;
@@ -12582,6 +12630,20 @@ pub const Lowering = struct {
                         const elem = info.vector.element;
                         return if (is_opt_chain) self.optionalOfFlattened(elem) else elem;
                     }
+                    // Tuple field access: numeric `t.0` or named `t.x`.
+                    if (info == .tuple) {
+                        const tup = info.tuple;
+                        if (std.fmt.parseInt(usize, fa.field, 10)) |idx| {
+                            if (idx < tup.fields.len)
+                                return if (is_opt_chain) self.optionalOfFlattened(tup.fields[idx]) else tup.fields[idx];
+                        } else |_| {}
+                        if (tup.names) |names| {
+                            for (names, 0..) |nm, i| {
+                                if (nm == field_name_id and i < tup.fields.len)
+                                    return if (is_opt_chain) self.optionalOfFlattened(tup.fields[i]) else tup.fields[i];
+                            }
+                        }
+                    }
                     // Check struct fields
                     const fields = self.getStructFields(obj_ty);
                     for (fields) |f| {

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