PR-014 - PBS Switch Wildcard Rule Enforcement

Briefing

Switch semantics do not currently enforce all wildcard constraints. This PR enforces: at most one wildcard arm, and rejection of mixed default and _ in the same switch.

Motivation

Wildcard ambiguity weakens deterministic static semantics and conformance.

Target

Switch expression semantic validation in flow analyzer.
Required static diagnostics for wildcard rule violations.

Scope

Enforce wildcard-count and wildcard-mixing rules.
Preserve existing selector/type compatibility checks.

Method

Track wildcard token form used per switch arm.
Emit diagnostics for duplicate wildcard and mixed wildcard spellings.
Keep existing exhaustiveness behavior.

Acceptance Criteria

Two wildcard arms are rejected deterministically.
default + _ in one switch is rejected deterministically.
Existing duplicate-pattern logic for non-wildcards remains unchanged.

Tests

Add tests for duplicate wildcard arms.
Add tests for mixed default/_ arms.
Run semantic control-flow suite.

Non-Goals

New switch syntax.
Runtime switch lowering changes.

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