feat: add fallible iterator implementations for explicit error handling

[digizeph]

Summary

This PR implements fallible iterators that expose parsing errors to users, addressing the current limitation where errors are silently skipped and only logged as warnings.

New Features:

• FallibleRecordIterator - Returns Result<MrtRecord, ParserErrorWithBytes> instead of skipping errors
• FallibleElemIterator - Returns Result<BgpElem, ParserErrorWithBytes> instead of skipping errors
• into_fallible_record_iter() and into_fallible_elem_iter() methods on BgpkitParser

Key Benefits:

• Users can now handle parsing errors explicitly instead of having them silently skipped
• Full backward compatibility maintained - existing iterators unchanged
• Better error visibility for debugging and data quality monitoring
• Follows Rust error handling best practices

Module Reorganization:

• Restructured iterator implementations into iters module with default and fallible submodules
• Cleaner separation of concerns between error-skipping and error-exposing iterators
• All trait implementations consolidated in iters/mod.rs

Test Plan

• All existing tests pass (246 tests)
• Added comprehensive test coverage for fallible iterators (4 new tests)
• Example code demonstrating usage patterns (examples/fallible_parsing.rs)
• Verified backward compatibility - no breaking changes
• Code formatting and clippy checks pass
• Build verification with and without default features

Usage Example

use bgpkit_parser::BgpkitParser;

let parser = BgpkitParser::new("updates.mrt").unwrap();
for result in parser.into_fallible_record_iter() {
    match result {
        Ok(record) => {
            // Process the record normally
        }
        Err(e) => {
            // Handle the parsing error explicitly
            eprintln\!("Error parsing record: {}", e);
        }
    }
}

Backward Compatibility

✅ No breaking changes - all existing APIs remain unchanged
✅ Default behavior preserved - standard iterators continue to skip errors
✅ Additive only - new functionality is opt-in via new methods