Refactor or-pattern type checking by category

[bradcypert]

Or-pattern checking now dispatches by literal, tag-shaped, and structural category. Literal alternatives constrain the scrutinee without constructing binding environments; tag and structural alternatives share explicit rules requiring identical binding names, unique names within each alternative, and unifiable binding types.

Issue #74 predates #75 (merged in PR #82), which enabled tag and structural alternatives. This refactor preserves those supported shapes and their existing rejection diagnostics rather than disabling them. Constructor heads still resolve through the ordinary pattern binder, including positional struct syntax.

Adds category unit coverage, execution cases for string/float/bool and nested/mixed alternatives, and a structural binding-type rejection fixture. Rebuilds the tracked compiler.

Validation:

• bootstrap/self-test (12 passed)
• bootstrap/corpus-check (including AddressSanitizer and leak checks)
• bootstrap/bootstrap-check
• bootstrap/check-seed
• bootstrap/fmt-check
• git diff --check

Closes #74