docs+rename: erase the reify name everywhere — stream is METATYPE

The compiler concept is declare/define (comptime type construction); the
old "reify" framing is gone from the entire repo.

- Rename: PLAN-REIFY → PLAN-METATYPE, CHECKPOINT-REIFY → CHECKPOINT-METATYPE,
  PLAN-POST-REIFY → PLAN-POST-METATYPE (both rewritten around declare/define);
  examples 0614/0615/0617 → comptime-metatype-* (+ their expected/ triplets),
  headers rewritten.
- Scrub reify from design/execution-evolution-roadmap.md (§7 step 3 contracts,
  §8.1, §9 decisions, §10 gates) → declare/define / comptime type construction.
- core.sx prelude pointer + parser.test.zig surface lock updated to the
  declare/define builtins (define(handle, info) -> Type; EnumInfo.name).

No behavior change; renamed examples match their renamed snapshots. Full
suite green (673), all unit tests pass. Zero `reify` tokens remain in
src/docs/sx/examples.

library/modules/std/core.sx

@@ -31,10 +31,11 @@ field_value_int :: ($T: Type, idx: i64) -> i64 #builtin;
 field_index :: ($T: Type, val: T) -> i64 #builtin;
 error_tag_name :: (e: $T) -> string #builtin;
 
-// Comptime type metaprogramming (`type_info` / `reify` / `field_type`) lives in
-// the on-demand `modules/std/meta.sx`, NOT here — declaring its data types in
-// the always-loaded prelude would intern them into every module's type table
-// and shift every `.ir` snapshot. Import `modules/std/meta.sx` to use reify.
+// Comptime type metaprogramming (`declare` / `define` / `type_info` /
+// `field_type`) lives in the on-demand `modules/std/meta.sx`, NOT here —
+// declaring its data types in the always-loaded prelude would intern them into
+// every module's type table and shift every `.ir` snapshot. Import
+// `modules/std/meta.sx` to construct or reflect types at comptime.
 
 // Call-site location, synthesized by the `#caller_location` directive when it
 // is a parameter's default value (ERR E4.1b). `process.exit` / `assert` use it