Extract type handler common rep

[FrozenLemonTee]

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[Copilot]

Pull request overview

This PR introduces an extensible “common representation” negotiation mechanism for mixed-representation operations, shifting type-handler common-rep selection from hardcoded std::common_type_t to customizable underlying traits, and updates tests/examples accordingly.

Changes:

• Add underlying::common_rep_traits + exported has_common_rep / common_rep_t APIs (defaulting to std::common_type_t when available).
• Update type policy handlers (compatible/transparent) to gate on has_common_rep and use common_rep_t.
• Add tests and example updates demonstrating default/common-rep customization and improve example file headers.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/underlying/traits.cppm	Adds common_rep_traits and exports has_common_rep / common_rep_t.
src/policy/impl.cppm	Updates type handlers to require has_common_rep and use common_rep_t.
src/primitive/traits.cppm	Refines module imports to submodules (policy.*, underlying.traits).
src/primitive/impl.cppm	Refines module imports to submodules (policy.traits, underlying.traits).
tests/basic/test_templates.cpp	Adds common-rep default/customization assertions and handler checks.
tests/basic/test_operations.cpp	Formatting adjustments; updates expectation for common_rep when disallowed.
examples/ex01_default_arithmetic.cpp	Adds example header comment block.
examples/ex02_type_policy.cpp	Adds example header comment block.
examples/ex03_value_policy.cpp	Adds example header comment block.
examples/ex04_error_policy.cpp	Adds example header comment block.
examples/ex05_concurrency_policy.cpp	Adds example header comment block.
examples/ex06_custom_underlying.cpp	Demonstrates custom common_rep_traits negotiation (plus header/docs).
examples/ex07_custom_policy.cpp	Adds example header comment block.
examples/ex08_custom_operation.cpp	Adds example header comment block.

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