Support pipe fallback in pivot components

[zzhang82]

/claim #1640

Summary

• Add | fallback within each colon-delimited --pivot component.
• Use the first non-empty field/tag value from left to right, eg --pivot 'mynotes|desc'.
• Preserve existing colon composition behavior, so expressions like acct:category|desc work per component.

Validation

• Built hledger-lib and hledger from source in Docker with GHC 9.6.7 using stack --stack-yaml stack96.yaml build hledger hledger-lib.
• Manually verified new fallback cases:
  • tag present uses tag value
  • absent tag falls back to desc
  • three-way fallback chooses first non-empty value
  • fallback works inside existing colon composition
  • all alternatives absent preserves empty-string pivot behavior
• Manually verified existing colon composition regression:
  • --pivot acct:kind:project still produces expenses:equipment:job1, expenses:equipment:job2, and expenses:fee:job2.
• Ran git diff --check.

[simonmichael]

Thanks for the PR. Currently this is blocked on my two latest comments on #1640.