fix(library): separate generated power pins

[neusse]

Summary

create_symbol passed raw multi-unit power_pins into the automatic rectangular builder. Pins with no supplied angle therefore landed at the same coordinate, so a generated VCC/GND power unit could short its own supply rails while the tool reported success.

This focused fix applies the existing layout_power_unit() helper when none of the power pins supplies an angle. Any explicit arrangement remains caller-controlled. After layout, duplicate resolved connection points are rejected with a structured power_pins error before the library is written. The response also derives power_pin_count from pins actually generated rather than echoing ignored request data.

Closes #458

Acceptance accounting

• Multi-unit power pins with no supplied angles use the existing top/bottom power-unit layout.
• Any explicitly positioned/angled power-pin arrangement is preserved, including multiple pins intentionally entering from the same side.
• Different pin numbers that resolve to one connection point, including equivalent 0/360-degree inputs, fail before the symbol library is written; legal stacked representations of the same physical pin number remain supported.
• Handler-level regressions inspect the generated dedicated power unit and cover default layout, explicit mixed arrangements, collision refusal, legal same-number stacking, and truthful output counts.
• Existing single-unit drawn and triangular-split behavior remains covered.
• No schema, tool-count, file-format, or dependency change.

Red/green and adversarial evidence

Before the fix:

cargo test -p konnect-core --lib multi_unit_power_pins_are_distributed_within_the_power_unit -- --nocapture
FAILED: VCC and GND both at (-2.54, 0.0)

cargo test -p konnect-core --lib power_pins_without_units_report_zero_written_pins -- --nocapture
FAILED: reported 2, expected 0

After the first fix, adversarial review exposed that unconditional layout reversed explicitly arranged pins. A second review exposed that duplicate raw angles are not sufficient evidence of a collision and that 0/360 degrees are equivalent. The final implementation preserves every arrangement containing an explicit angle, and validates the resolved connection points instead. A final spec pass showed that omitted-angle same-number stacks also have to share a layout slot. The final implementation now preserves that ordinary case as well, and the regression asserts the two resolved anchors remain identical.

Complete local gate on this head:

cargo fmt --all -- --check                                      pass
cargo clippy --workspace --locked --all-targets -- -D warnings  pass
cargo test --workspace --locked --lib --tests                   pass
cargo test --workspace --locked --doc                           pass

Environment-dependent checks requiring a live KiCad GUI remain intentionally ignored by the repository test suite. This change exercises the real public handler and parses the generated .kicad_sym; KiCad CLI has no standalone symbol-library validation command.

Branch, compatibility, and risk

• Base: current upstream/main at 5afdcbec76b3a8e55f87da778411ee09a98ae45b.
• Dependencies: none; one unique commit.
• Compatibility: request and response schemas are unchanged. Correctly generated coordinates replace unsafe coincident defaults; explicit arrangements remain unchanged; invalid collisions now fail atomically.
• Risk is confined to generated symbol power-pin placement, collision validation, and count reporting. Rollback is reverting the single commit.