Follow up: harden Class-A I2C registry metadata and obsolete-part handling #373

Follow-up from #322

Harden the registry metadata and validation introduced with the Class-A I2C parts:

1. Make the CH340C V3 decoupling requirement machine-checkable for 5 V supply mode instead of documenting the condition only in prose. The model should prevent a 5 V design from silently omitting the required V3 capacitor while preserving the valid 3.3 V connection mode.
2. Add a regression test proving obsolete registry entries cannot be selected silently. In particular, 24lc256_eeprom has an obsolete/incorrect pin model and should resolve to or require migration to 24lc256_soic8.
3. Replace the generic reseller URLs for header_1x5 and header_1x12 with authoritative manufacturer documentation where available, and retain the review/provenance metadata.

Acceptance criteria:

• Registry qualification and focused tests cover the conditional CH340C V3 requirement.
• A design referencing an obsolete part receives an explicit migration/error result rather than using the stale pin model.
• Header provenance points to authoritative documentation and all registry tests remain green.