green(reify): field_type($T, i) -> Type over the type table

REIFY Phase 2.1. fieldTypeOf (lower/generic.zig, re-exported on Lowering)
returns the i-th member type of T: struct field / tagged-union + union
variant payload (.void for a tagless variant) / tuple element / array +
vector element. Out-of-range and memberless types poison to .unresolved
with a loud diagnostic (never a silent default). Wired into
resolveTypeCallWithBindings (replacing the Phase-2 bail); since it folds
to a TypeId at lower time it composes inside type_eq / type_name / any
type-arg slot.

examples/0616 green: struct fields (name via field_name + type via
field_type), type_eq fold, tagged-union payloads incl. quit -> void.
Suite green (672 examples, 447 unit).

type_info($T) -> TypeInfo (reflect into a value, inverse of reify) is
NOT done — still bails loudly; it's the larger Phase 2.2 step (widen the
TypeInfo data model + comptime value construction). Plan/checkpoint updated.

examples/expected/0616-comptime-field-type.exit

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

examples/expected/0616-comptime-field-type.stderr

@@ -0,0 +1 @@
+

examples/expected/0616-comptime-field-type.stdout

@@ -0,0 +1,9 @@
+Point has 3 fields
+  0: x : i64
+  1: y : f64
+  2: on : bool
+field 0 is i64: true
+field 1 is f64: true
+Msg.num  payload: i64
+Msg.tag  payload: bool
+Msg.quit payload: void