Allow miners to post direction specific rates

[LandynDev]

Summary

Miners post a single bilateral rate that applies to both swap directions. But tx fees are asymmetric across chains — sending BTC costs more than receiving it. A single rate forces miners to average out that cost, meaning users get a worse rate on the cheap direction.

This adds direction-specific rates so miners can price each direction honestly (e.g., 340 TAO/BTC for BTC->TAO, 350 TAO/BTC for TAO->BTC). The spread captures the tx fee differential.

Approach

Commitment format bumped to v3 with two rate fields. The correct rate is selected based on swap direction at initiation time. The entire backend (validator verification, miner fulfillment, smart contract, voting) is unchanged — each swap on-chain still stores one rate, it's just the right one for that direction now.

Test plan

• 54/54 unit tests pass (17 new commitment parsing tests including normalization proof)
• Lint clean (ruff format + check)
• E2E suite 02: 21/21 pass (full swap lifecycle)
• Manual CLI verification of all directional rate flows

Reviewer experience

Start the dev environment and explore these flows:

Post a pair with directional rates (as miner):

alw --network local --netuid 2 --wallet.name test-miner-1 --wallet.hotkey test-miner-1 \
  miner post btc <btc_addr> tao <tao_addr> 340 360

Post same rate both ways:
alw --network local --netuid 2 --wallet.name test-miner-1 --wallet.hotkey test-miner-1 \
  miner post btc <btc_addr> tao <tao_addr> 345

Check miner status shows both rates:
alw --network local --netuid 2 --wallet.name test-miner-1 --wallet.hotkey test-miner-1 miner status

View rates as a user (separate columns per direction):
alw --network local --netuid 2 view rates
alw --network local --netuid 2 view miners

Initiate swap and verify correct rate is used:
alw --network local --netuid 2 --wallet.name test-user --wallet.hotkey test-user swap now
# Choose BTC->TAO → should show rate 340
# Choose TAO->BTC → should show rate 360