spel init produces a scaffold that does not build out of the box

[vpavlin]

Summary

Running `spel init ` in a fresh directory, then `make build` immediately, fails with unrelated-looking errors. Two independent problems stack:

1. Stale default `--spel-tag`. `spel-cli/src/init.rs:287` pins `spel-framework` to `tag = "v0.2.0-rc.1"`. That tag's `spel-framework/Cargo.toml` resolves `nssa_core` via `rev = "ffcbc15972adbf557939bf3e2852af276422631b"`, but the scaffold's own Cargo.toml pins `nssa_core = { tag = "v0.2.0-rc1" }` (no dot, and a strictly newer commit — `35d8df0`). Both end up in the build tree, and `#[lez_program]` macro expansion fails with type mismatches like:

   ```
   error[E0061]: this function takes 5 arguments but 3 arguments were supplied
   --> #[lez_program] macro expansion
   expected [u32; 8], found Vec
   argument Support dynamic/arg-based PDA seeds #2 of type Option<[u32; 8]> is missing
   ```

   Subsequent spel rc tags (`v0.2.0-rc.2`, `v0.2.0-rc.3`) fix this by pinning `nssa_core = { tag = "v0.2.0-rc1" }` — matching the scaffold's pin. Note that the final `v0.2.0` GA tag regresses to the `ffcbc15` rev, so the right default to bump to is `v0.2.0-rc.3`, not GA.

2. `ruint@1.18.0` MSRV clash. The RISC-Zero docker image `cargo risczero build` uses ships rustc 1.88.0-dev. `ruint@1.18.0` (transitive via `risc0-binfmt → risc0-build → risc0-zkvm`) now requires rustc 1.90. The user sees:

   ```
   error: rustc 1.88.0-dev is not supported by the following package:
   ruint@1.18.0 requires rustc 1.90
   ```

   Workaround: `cargo update -p ruint --precise 1.17.0 --manifest-path methods/guest/Cargo.toml` before `make build`. Neither the scaffold nor any doc mentions this.

Repro

```bash
spel init demo
cd demo
make build # fails with ruint MSRV error

cargo update -p ruint --precise 1.17.0 --manifest-path methods/guest/Cargo.toml
make build # fails with two-nssa_core errors from #[lez_program]
```

With `spel init --spel-rev refs/heads/main demo` the second failure goes away (main has the aligned pin). With `--spel-tag v0.2.0-rc.3` it should also go away.

Proposed fix

• Bump the default `spel_ref` in `spel-cli/src/init.rs` from `tag = "v0.2.0-rc.1"` to `tag = "v0.2.0-rc.3"`.
• Pin `ruint` in the scaffolded `methods/guest/Cargo.toml` (or note the `cargo update` workaround in the scaffolded README) until the RISC-Zero docker image picks up a newer rustc. Alternatively, patch the scaffolded lockfile with `[patch.crates-io] ruint = …` so users don't have to know about it.

Impact

The tutorial (docs/tutorial.md) says "Step 1: `spel init my-counter`, Step 4: `make build`". Today that fails on a clean machine without any hint that the defaults need changing. Found during the tutorial walk that produced #138.

cc @fryorcraken