fix(stableswap): reorder peg sources to match sorted assets during pool creation

[nicolad]

When creating a stableswap pool with pegs, assets are sorted by ID for consistent ordering, but the corresponding peg sources and current pegs were not reordered to match. This caused pegs to be associated with the wrong assets after sorting.

The fix creates an index mapping before sorting and uses it to reorder both peg_info.source and peg_info.current arrays to maintain correct asset-to-peg associations.

Related Issue

Fixes: #1164

Motivation and Context

When creating a pool with assets [3, 1, 2] and pegs [Peg3, Peg1, Peg2]:

• Assets get sorted to [1, 2, 3]
• Pegs stayed as [Peg3, Peg1, Peg2] (wrong!)
• Result: Asset 1 incorrectly gets Peg3 instead of Peg1

This caused incorrect price calculations and potential trading failures in stableswap pools using pegged assets.

How Has This Been Tested?

• Added 7 comprehensive tests in pallets/stableswap/src/tests/peg.rs covering:
  • Unsorted assets with pegs
  • Reverse order assets
  • 2-asset and 5-asset pools
  • Complex ordering scenarios
  • Index-based peg retrieval validation
  • Asset-peg association verification
• All 170 existing stableswap tests pass
• Formatting and clippy checks pass

Checklist

• I have updated the documentation if necessary.
• I have added tests to cover my changes, regression test if fixing an issue.
• This is a breaking change.

[nicolad]

Please have a look here
@martinfridrich
@Roznovjak