Clarify differential pair ambiguity warnings - #3017

Follow-up to #3011.

What changed

• Report a named differential-pair trace with more than two terminal pins as an ambiguous trace name.
• Include the pair, connection property, trace name, and complete terminal-pin list.
• Report selector and incomplete-connection failures factually as expected exactly 2 terminal pins, found N.
• Remove all advice to delete connections or switch to a pin selector.
• Remove the unnecessary singular/plural message construction.

Why

Core does not ignore or rewrite the differential-pair connection. The diagnostic should state the topology problem without prescribing a circuit or selector change. A named trace is ambiguous when its resolved connection contains more than two terminal pins; other invalid counts are simply not point-to-point.

Developer impact

Users receive concise, non-prescriptive diagnostics that identify the actual connection problem and list the affected pins.

Validation

• Focused differential-pair warning tests: 3 passing, 10 assertions
• bunx tsc --noEmit
• Biome checks on all changed files
• git diff --check
• Three independent review passes covering wording, edge cases, and minimal-diff quality