fieldcheck: add uk/ca/de postal_code semantic type

[wufangyong973]

closes #512

each country gets its own compact regex: UK is 1-2 letters plus a district plus the inward, Canada alternates letter/digit, and the German PLZ is five digits. normalisation (uppercase + strip) happens in the per-cell step, the vectorised pre-screen only matches the space-free canonical form, so spaced or lowercase values fall through to the per-cell check and get re-decided there, same rhythm as email/url.

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[JohnnyWilson16]

Thanks for working on this, @wufangyong973! The helper functions, normalization, and integration into _suspect_rows and validate_fields are well structured.

However, there is a functional bug in the UK postcode regex that causes valid two-digit districts to fail:

Issue: Two-digit UK districts fail validation

In src/freshdata/fieldcheck.py:93:

_UK_POSTAL_RE = re.compile(r"^[A-Z]{1,2}[0-9][A-Z]{0,1}[0-9][A-Z]{2}$")

Notice that [A-Z]{0,1} only permits a second letter in the outward code (e.g. W1A, SW1A). It rejects postcodes with a second digit (standard formats A99 and AA99), such as:

• B33 8TH (Birmingham) -> False
• SN10 1AA (Swindon) -> False
• M60 1NW (Manchester) -> False
• OX14 4SE (Oxford) -> False

Interestingly, your docstring on line 91 explicitly lists B33 and SN10 as examples!

Fix:

Update _UK_POSTAL_RE and _POSTAL_COMPACT_RE to allow an optional second digit or letter:

_UK_POSTAL_RE = re.compile(r"^[A-Z]{1,2}[0-9][0-9A-Z]?[0-9][A-Z]{2}$")

_POSTAL_COMPACT_RE = re.compile(
    r"^([A-Z]{1,2}[0-9][0-9A-Z]?[0-9][A-Z]{2}|[A-Z][0-9][A-Z][0-9][A-Z][0-9]|[0-9]{5})$"
)

Please also add test cases in tests/test_fieldcheck.py covering two-digit districts like B33 8TH and SN10 1AA. Once updated, this will be good to go!