docs(pair): clarify numerator/denominator naming and document error conditions

[thedavidmeister]

Closes #92
Closes #103

Summary

Two related readability issues in LibOpFtsoCurrentPricePair.sol:

#103 — inverted naming relative to call order

The pair op fetches the denominator symbol (symbolB) first via a memory-truncation trick, then the numerator (symbolA) second. The old variable names outputsB/priceB for the first call and outputsA/priceA for the second call follow the mathematical role of each price but are surprising when reading top-to-bottom code. Renames to denominatorOutputs/denominatorPrice and numeratorOutputs/numeratorPrice, and adds a comment explaining that the denominator is fetched first.

#92 — missing error documentation

run() NatSpec did not document:

• The three inherited FTSO errors (InactiveFtso, StalePrice, PriceNotFinalized) that can be raised from either price leg.
• That the denominator leg is evaluated first, so errors on symbolB abort before symbolA is queried.
• The DivisionByZero revert from LibDecimalFloat when the symbolB USD price is zero.

Adds a @dev block covering all of these.

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📥 Commits

Reviewing files that changed from the base of the PR and between cb20d03 and 4094f02.

📒 Files selected for processing (1)

• src/lib/op/LibOpFtsoCurrentPricePair.sol

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-06-30-issue-92-103-pair-natspec-rename

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