[Enhancement] Implement #[fixture] proc-macro via crucible-macros with reset() method generation

[ekelemepraise-code]

Summary

The README documents a #[fixture] attribute macro:

#[fixture]
pub struct AmmFixture { ... }

But crucible-macros/src/lib.rs does not implement this macro. Users who try to use it get a compile error.

Expected Behaviour

The macro should:

1. Require AmmFixture::setup() -> Self to be implemented manually
2. Auto-generate a reset(&mut self) method that calls *self = Self::setup()
3. Optionally generate a Default impl that calls Self::setup()

Implementation Plan

In crucible-macros/src/lib.rs:

#[proc_macro_attribute]
pub fn fixture(_args: TokenStream, input: TokenStream) -> TokenStream {
    // Parse struct definition
    // Inject reset() method
    // Re-emit struct unchanged
}

Acceptance Criteria

• #[fixture] attribute macro compiles correctly on a struct
• reset() method is generated
• Macro emits a clear error if struct does not implement setup() -> Self
• Trybuild tests added in crucible-macros/tests/
• README example updated to show the macro working