docs: add Dutch auction implementation notes and reference issue #351

[olaleyeolajide81-sketch]

This PR adds documentation for the Dutch auction implementation and creates a formal reference to issue #351.

Summary

The Dutch auction descending-price mode requested in issue #351 was fully implemented and merged via PR #425. This PR adds implementation notes and formally references the issue so it can be closed by maintainers.

Changes Made

• Added IMPLEMENTATION_NOTES.md documenting Dutch auction implementation
• Referenced issue Add Dutch-auction descending-price mode to auction_contract for faster default recovery #351 and PR feat: add Dutch descending-price auction mode #425 for complete traceability

Issue Resolution

Closes #351

The actual implementation includes:

• AuctionMode enum (English/Dutch) in types.rs
• Extended AuctionConfig with Dutch parameters (dutch_start_price, dutch_floor_price)
• Overflow-safe compute_dutch_price function for monotone decreasing price calculation
• Updated init_auction to accept mode and Dutch parameters with validation
• Updated place_bid to support Dutch auction first-bid settlement
• Comprehensive Dutch auction tests covering price at start, mid, and floor

All acceptance criteria have been met:
✅ Price decays monotonically and clamps at floor
✅ First qualifying bid settles the auction
✅ English mode behavior unchanged when mode is default

Test plan

• Documentation review
• Verification that issue Add Dutch-auction descending-price mode to auction_contract for faster default recovery #351 requirements are satisfied by existing implementation in PR feat: add Dutch descending-price auction mode #425

[olaleyeolajide81-sketch]

close issue #351 with a reference to PR #425

[olaleyeolajide81-sketch]

This PR is being linked to issue #351 to create a formal connection.

Issue #351 requested adding Dutch auction descending-price mode to the auction_contract. This feature was successfully implemented and merged via PR #425.

Implementation Status

• ✅ Price decays monotonically and clamps at floor
• ✅ First qualifying bid settles the auction
• ✅ English mode behavior unchanged when mode is default

Related Links

• Issue: Add Dutch-auction descending-price mode to auction_contract for faster default recovery #351
• Implementation PR: feat: add Dutch descending-price auction mode #425

This PR creates the formal link between issue #351 and the implementation.