Digit-level confusions are missing, and those are the ones on card and account numbers

[royalpinto007]

_CONFUSABLE covers the seven -teen/-ty pairs. Those are the classic case and worth having first, but the confusions that cost the most on a real phone line are in digit strings, and none of them are listed:

• "oh" and "zero" read back in a card or account number
• "a" and "eight", which is the same one-phoneme problem as fifteen/fifty
• "to", "two" and "too" where an amount or a quantity follows
• "double seven" and "seventy seven", and the same for any repeated digit, which is how a lot of people read numbers aloud
• letter confusions on a phone line: B, D, E, P, T, V and Z are famously indistinguishable over narrowband audio, and they turn up in reference codes and postcodes

A misheard reference code is not a small error. It routes the call to the wrong account and the agent proceeds with complete confidence, which is exactly the class of failure check_misheard is for.

Acceptance

• New pairs added with a comment explaining the acoustic reason, matching the style of the existing list
• A fixture call containing a misread account number produces a finding
• No new false positives on the existing good_call.json fixture

Good first issue: additive, and the existing fixtures pin the current behaviour.