Add Enumeration::values() static slice for variant iteration

[iainmcgin]

Partially addresses #23 (at least, the most common use case for enum attributes).

Generated enums now expose Enumeration::values(), returning a &'static [Self] slice of every primary variant in proto declaration order. This subsumes the most common reason users reach for strum::EnumIter (the example in #23) and additionally enables .contains(), .len(), indexing, binary search, etc.

Example

For

enum Status {
  UNSPECIFIED = 0;
  ACTIVE = 1;
  INACTIVE = 2;
}

users can now write

for v in Status::values() {
    println!("{:?} = {}", v, v.to_i32());
}
assert!(Status::values().contains(&Status::ACTIVE));
assert_eq!(Status::values().len(), 3);

Behavior

• Aliases (additional names sharing an existing value when option allow_alias = true) are skipped — they remain accessible as the existing pub const aliases on the enum, but they aren't enum variants in Rust so they don't belong in values().
• Order matches the .proto declaration order, which is also the order from_i32 resolves to (so Status::values()[i].to_i32() lines up with from_i32 for unique values).

Trait change

Enumeration::values is added with a default implementation returning an empty slice so out-of-tree consumers implementing Enumeration against an older buffa version continue to compile. Codegen unconditionally overrides the default with the real impl.

Tests

• New unit tests in tests/generation.rs: test_enum_values_emits_static_slice_in_declaration_order and test_enum_values_skips_aliases.
• Workspace tests, clippy -D warnings, rustfmt, and markdownlint clean.
• task gen-wkt-types and task gen-bootstrap-types run; the resulting drift in buffa-types/src/generated/ and buffa-descriptor/src/generated/ is just the new values() impls.

Follow-up

A separate PR will add enum_attribute (the symmetric inverse of message_attribute from #44) for users who want to inject Rust attributes onto enums but not messages.