Error on enums with explicit discriminant values in SpacetimeType

[clockwork-labs-bot]

Fixes #4618

SATS encoding assigns variant tags by declaration order, ignoring Rust discriminant values. An enum like:

#[derive(SpacetimeType)]
enum Foo {
    A = 5,
    B = 10,
}

would silently encode A as tag 0 and B as tag 1, which causes a mismatch between server and client enum values. This PR makes the SpacetimeType derive macro emit a compile error when it encounters explicit discriminant values:

error: explicit discriminant values are not supported by SpacetimeDB; SATS assigns variant tags by declaration order, so discriminant values would be silently ignored
  --> src/lib.rs:3:7
   |
3  |     A = 5,
   |       ^

Internal enums that need explicit discriminants for non-codegen reasons (e.g. as u8 casts) can opt out with #[sats(allow_discriminants)].

Includes UI tests for:

• Enums with all explicit discriminants (error on each)
• Mixed implicit/explicit discriminants (error only on explicit)
• Plain enums with no discriminants (compiles fine)