docs: add YIP-78 (Sonne hack compensation)

[BobTheBuidler]

Summary

• Add YIP-78 describing partial compensation for Sonne hack-affected vault users.

Rationale

• Capture the proposal text with required structure and citations.

Details

• Adds inline references and a References section for external artifacts

[murderteeth]

Issues

• yip-78.md:93-94 - The References section only lists 2 entries (depositor gist + merkle distributor), but the Background section cites [1]-[6] inline (yAudit Sonne report, Rekt.news article, Discord OP rewards announcement, yvUSDT vault link, yvDAI vault link, loss data screenshot). These references are all present in the original forum post but missing here, leaving the inline citations dangling.

Please add the missing references from the source governance forum post.

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This review was conducted using the review-pr skill.

[BobTheBuidler]

Addressed the requested citation gaps in commit 815e216.

Fixed:

• Added missing Background sources so [1]..[6] are all defined in References.
• Added screenshot traceability with both forum permalink and original media URL.
• Resolved numbering collision by moving process links to [7] and [8].
• Verified citation set is consistent with no dangling refs ([1]..[8]).

Validation:

• bundle exec yip_validator YIPS/yip-78.md => total:1, valid:1, invalid:0, errors:0