docs(design): execution-model roadmap + reify implementation stream

Add the async-first execution-model roadmap (comptime JIT spine, colorblind
fibers/Io, atomics, hot-reload) with all seven decisions resolved and
three-way reviewed, and carve the first stream: comptime type_info/reify
(PLAN-REIFY + checkpoint) — the codebase-validated foundation for channel
result types and race's synthesized tagged union.

current/CHECKPOINT-REIFY.md

@@ -0,0 +1,32 @@
+# CHECKPOINT-REIFY — comptime `type_info` / `reify` (async-first foundation, step 3)
+
+Companion to [PLAN-REIFY.md](PLAN-REIFY.md). Update after every step (one step at a
+time, per the cadence rule).
+
+## Last completed step
+**None — stream just carved.** Design validated (3 codebase reviewers; all five reify
+contracts confirmed feasible). No code written yet.
+
+## Current state
+- The plan + the five locked contracts exist in `PLAN-REIFY.md`; design-of-record is
+  `design/execution-evolution-roadmap.md` §7 step 3 + §8.1.
+- **Nothing built.** `reify`/`type_info`/`field_type` do not exist in the compiler.
+- Confirmed against the source (anchors in the plan): type minting via
+  `intern`/`internNominal` is programmatic and AST-free; type-fns memoize by mangled
+  name; enum codegen is fully type-table-driven (zero AST coupling); recursive
+  forward-declaration (reserve→complete) already exists for source types.
+
+## Next step
+**Phase 0.0 (lock):** add `TypeInfo`/`EnumInfo`/`EnumVariant` data types + bodyless
+`#builtin` decls for `reify`/`type_info`/`field_type` to `library/modules/std/core.sx`
+(parsed, unimplemented → loud bail), with a unit test that the decls parse. Then 0.1
+(xfail: `examples/06xx-comptime-reify-enum.sx`) → 0.2 (green: implement `reify(.enum_)`).
+
+## Known issues
+None yet.
+
+## Log
+- **Stream carved.** Selected as the first async-first foundation: `reify` gates both
+  channel result types (`RecvResult($T)`) and `race`'s synthesized union, is fully
+  validated (3 reviewers), and is a self-contained compiler/type-system feature
+  testable in isolation (`06xx` comptime). Generic-enum syntax dropped in its favor.