refactor: encapsulation cleanup (send/respond_to?) and complete performance TODOs

[ronaldtse]

Summary

This PR completes two major workstreams:

1. Encapsulation Cleanup

• send() → public_send(): All dynamic dispatch now respects method visibility
• respond_to? → is_a?: Type checks use explicit type comparison instead of duck typing
• Test doubles: Updated anonymous classes to inherit from Lutaml::Xml::Namespace for proper type checking
• Documented edge cases: respond_to? kept where method is dynamically added (e.g., element_order on non-Serialize models)

2. Performance TODOs (all verified and removed)

TODO.remaining-perf (7 items — all verified DONE):

• 01: Truly lazy namespace collection — stores element reference instead of eager recursive walk
• 02: Short-circuit UninitializedClass in XML path (key_value path uses new with different nil semantics)
• 03: Replace respond_to? with type hierarchy — removed redundant is_a?(XmlElement) checks on children
• 04: Thread resolved type through call chain, cache resolver in Attribute
• 05: Concurrent::Map for @mappings (not compute_if_absent due to recursive deadlock risk)
• 06: Cache prefixed element names, reduce options.except/merge allocations
• 07: Thread instance_is_serialize flag instead of checking is_a?(Serialize) 8+ times per element

TODO.pi-phase1 (6 items — 4 verified DONE, 2 correctly deferred):

• 1: Multi-adapter roundtrip tests ✓
• 2: Hash attribute default nil handling ✓
• 3: Value map/transform for PIs — deferred (no use case)
• 4: Inline processing instructions — deferred (requires mixed_content)
• 5: RuboCop compliance ✓
• 6: Documentation ✓

Test Results

• 4931 examples, 0 failures
• All 3 remaining RuboCop offenses are pre-existing (block length in transformation.rb, safe nav in adapter_resolver.rb, verifying doubles in optimization_spec.rb)

Files Changed

• 81 files changed, 706 insertions, 381 deletions