Gleam: | alternatives in match arm patterns (ILO-411)

[danieljohnmorris]

Summary

• Adds Pattern::Or(Vec<Pattern>) to the AST for pipe-separated match alternatives
• Parser: parse_match_arm now collects |-separated patterns into Pattern::Or
• VM: compiles Pattern::Or by checking each alternative; matches if any hits
• Interpreter, verifier, formatter, and Python codegen all handle the new variant

Example

?x{"a"|"b":"found";_:"miss"}

Matches both "a" and "b" with the same arm body. Three or more alternatives are also supported: 1|2|3:"low".

Test plan

• vm_or_pattern_first_alt_matches — first alternative fires
• vm_or_pattern_second_alt_matches — second alternative fires
• vm_or_pattern_no_alt_matches — falls through to wildcard
• vm_or_pattern_three_alts — three alternatives, middle one matches
• parse_match_or_pattern_two_alts — AST shape correct for 2 alts
• parse_match_or_pattern_three_alts — AST shape correct for 3 alts
• Full test suite: 3373 + 399 tests pass, 0 failures

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 58.57988% with 70 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/codegen/python.rs	0.00%	33 Missing ⚠️
src/vm/mod.rs	78.49%	20 Missing ⚠️
src/interpreter/mod.rs	0.00%	6 Missing ⚠️
src/parser/mod.rs	81.25%	6 Missing ⚠️
src/verify.rs	0.00%	4 Missing ⚠️
src/codegen/fmt.rs	0.00%	1 Missing ⚠️

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