refactor: remove proc-macro-error2 dependency

[ya7010]

Summary

Removes the proc-macro-error2 dependency to reduce reliance on external crates whose maintenance status is uncertain, and unifies all derive-macro error handling under the existing crate::Error / to_compile_errors() mechanism.

• Made SingleIdentPath::new / NamedField::new / UnnamedField::new return Result<_, crate::Error>, completely eliminating panic-based abort! (no panic path remains other than parse_macro_input!)
• Changed collect_serde_rename_map to return Result<RenameMap, crate::Errors>, propagating with ? at its callers (named_struct_derive / enum_derive)
• Removed the #[proc_macro_error] attribute and the dependency declarations from Cargo.toml
• Error messages and spans are identical to before; since errors now flow through the existing Errors collection mechanism, multiple errors can be reported at once

Under the hood, proc-macro-error2 is just a wrapper that catches abort! panics via catch_unwind and renders them as compile_error!. This crate already has an equivalent Result-based mechanism, so the dependency could be replaced without adding any custom implementation.

Closes #112

Validation

• cargo build --workspace — passes
• cargo test --workspace --all-features — all 32 test suites pass
• cargo fmt -- --check / cargo clippy --workspace --all-features — zero warnings
• cargo tree -p serde_valid_derive — no remaining reference to proc-macro-error2
• Manual check: compiled a test crate containing #[validate(foo::bar)] and confirmed the same span-accurate error output as before:

  error: Path(='foo::bar') must be single ident path.
   --> src/main.rs:5:16
    |
  5 |     #[validate(foo::bar)]
    |                ^^^
  

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