Reject non-finite fixture amounts

[rogerchappel]

Summary

• reject non-finite invoice and payment amounts during fixture normalization
• report the source CSV path, row, and amount field through LedgerpetError
• cover library and CLI failures, including the guarantee that invalid fixtures create no report directory
• document that finite decimals and zero are supported

Commits

• fix: reject non-finite fixture amounts
• test: cover malformed fixture amounts
• docs: define fixture amount requirements

Verification

• npm run release:check
• git diff --check origin/main...HEAD
• git log --format="%h %an %ae %cn %ce" origin/main..HEAD (all commits: Roger Chappel miscanalysis@gmail.com)