lang F1 6: (..sources) materializes a pack into a protocol-typed tuple field

lowerTupleLiteral now coerces/erases each spliced spread element to the
contextual target tuple field type (computed even when a spread is present,
indexed by output position). New coerceOrErase: protocol target -> xx-erase
via buildProtocolErasure, else coerceToType. So c.sources = (..sources) on a
(..VL(Ts)) field erases each concrete pack element to its VL(Ti) slot.

examples/210 (build(IntCell, StrCell) -> 10 hi). 245 examples + unit green.

examples/210-pack-to-protocol-tuple.sx

@@ -0,0 +1,28 @@
+// Phase 6 — `c.sources = (..sources)`: materialize a pack into a
+// protocol-typed tuple field, erasing each concrete pack element to the field's
+// protocol slot. The pack `..sources: VL` holds concrete cells; `(..sources)`
+// into a `(..VL(Ts))` field `xx`-erases each to its `VL(Ti)` value.
+
+#import "modules/std.sx";
+
+VL :: protocol(T: Type) { get :: () -> T; }
+IntCell :: struct { v: s64; }
+StrCell :: struct { s: string; }
+impl VL(s64) for IntCell { get :: (self: *IntCell) -> s64 => self.v; }
+impl VL(string) for StrCell { get :: (self: *StrCell) -> string => self.s; }
+
+Combined :: struct($R: Type, ..$Ts: []Type) {
+    sources: (..VL(Ts));
+    value:   $R;
+}
+
+build :: (..sources: VL) -> void {
+    c : Combined(s64, ..sources.T) = ---;
+    c.sources = (..sources);                          // pack → tuple, per-element erase
+    print("{} {}\n", c.sources.0.get(), c.sources.1.get());
+}
+
+main :: () -> s32 {
+    build(IntCell.{ v = 10 }, StrCell.{ s = "hi" });   // 10 hi
+    0;
+}