dev: add selfhosting/cranelift workspaces; jit: add return materialization, ret_slot, hostcall arity pad, and dbg imports for diagnosis

dev/cranelift/README.md

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+Cranelift JIT/AOT Dev
+
+Focus: Cranelift JIT‑direct and AOT (object emit + link with `libnyrt.a`).
+
+Quick AOT Smoke
+
+- Build core (once):
+  - `cargo build --release --features cranelift-jit`
+- Lower to object + link with NyRT:
+  - `NYASH_DISABLE_PLUGINS=1 tools/aot_smoke_cranelift.sh apps/smokes/jit_aot_string_min.nyash app_str`
+- Run app:
+  - `./app_str`
+
+Useful env toggles
+
+- `NYASH_JIT_DUMP=1`: show JIT lowering summary
+- `NYASH_JIT_TRACE_LOCAL=1`: trace local slot loads/stores
+- `NYASH_JIT_TRACE_RET=1`: trace return path
+- `NYASH_JIT_TRACE_LEN=1`: trace string/any len thunks
+- `NYASH_JIT_DISABLE_LEN_CONST=1`: disable early const‑fold for String.length
+
+Notes
+
+- For AOT linking: requires `libnyrt.a` from `crates/nyrt` (built by `cargo build --release`).
+- Use `target/aot_objects/` as scratch; keep per‑experiment subfolders if needed.
+