fix(schematic): respect intentional unconnected pins

[nordic-style]

Summary

Stop validate_component_connections from reporting intentionally unconnected symbol pins and pins belonging to other units, and make the existing ignore_power_pins option effective.

Refs #182

Approach

• Preserve each library pin's KiCad electrical type while parsing embedded symbols.
• Exempt pins typed no_connect as well as ordinary pins carrying a schematic no-connect marker.
• Honor ignore_power_pins for power_in and power_out pins.
• Use the existing unit-aware pin extractor so a placed unit does not inherit another unit's pins.
• Add pin_type to each reported finding and clarify the public tool description.

This is the validator-specific part of #182. #272 covers the remaining analysis/export/review paths and #273 covers reference-based component mutations.

Compatibility and safety

The request schema is unchanged. Findings gain an additive pin_type field. Results intentionally stop containing library-declared NC pins, ignored power pins, and pins from unplaced units.

This validator is read-only and does not alter the schematic.

Validation

• cargo fmt --all -- --check: pass
• cargo clippy -p konnect-core -p konnect-sexp --locked --all-targets -- -D warnings: pass
• cargo test -p konnect-sexp --locked: 134 unit tests, 8 property tests, and 6/7 doc tests pass (1 ignored)
• cargo test -p konnect-core --locked --lib midwire_pin_tests: 3 pass, covering declared NC, power filtering, selected units, and mid-wire junction behavior
• Workspace core tests reach 544 passing tests; the same three local update_symbols_from_library_* failures reproduce on origin/main because the installed KiCad Device:R library shadows the fixture
• Real schematic comparison: the released validator reported 185 findings, predominantly pins named NC; the patched validator reported 0 on the same saved input

Review checklist

• The diff is focused and contains no generated output, personal data, or unrelated cleanup.
• New names follow docs/NAMING_CONVENTIONS.md; public renames include compatibility handling.
• New behavior and failure paths have regression coverage.
• File mutations are atomic and preserve unrelated content. (Read-only.)
• IPC mutations verify the requested board and do not leave partial batches. (No IPC mutation.)
• Tool counts are unchanged.

[mixelpixx]

This and #280 add the identical ignore_power_pins filter to validate_component_connections — same lines, two different issue framings. See the series note on #285: please keep one. My preference is this one (#182 framing, smaller diff), with #280 reduced to whatever non-duplicate remainder it has after rebasing on #272.