feat: add Czech postal geocoding

docs/geocode-CZE.md

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+# Czechia postal geocoding (CZE)
+
+## Source and license
+
+Data: [GeoNames CZ postal export](https://download.geonames.org/export/zip/CZ.zip),
+retrieved 2026-11-04. The [export directory](https://download.geonames.org/export/zip/)
+listed the archive as 2026-11-04 03:53 CET. This is the export date, not a verified
+survey date. Source text SHA-256:
+`0534ebad1470188b4323a5ece68786e261cab07027082672fa782ab74c7553d3`.
+
+Credit: [GeoNames](https://www.geonames.org/), licensed under
+[CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).
+The [source readme](https://download.geonames.org/export/zip/readme.txt) permits
+redistribution with attribution and provides the data without a completeness or
+accuracy warranty. This file documents our normalization and aggregation changes.
+
+## Coverage and lookup
+
+The export contains 15,507 locality rows and 2,694 unique full five-digit Czech
+postcodes. The output contains 2,694 records. Coverage is source-limited; no complete
+national coverage claim is made. No missing code gets a fallback point.
+
+CZE accepts `11000` and `110 00`, with surrounding whitespace. It rejects prefixes,
+extra text, country prefixes, and malformed codes. Keys remain strings. No leading
+zero is removed or added; the export has no leading-zero postcodes. Unknown codes
+return `undefined`. Coordinates retain the existing `[longitude, latitude]` string
+return format. Other countries keep their current normalization.
+
+For each postcode, the generator removes duplicate numeric coordinate pairs,
+sorts them, and takes their unweighted arithmetic mean. It rounds to six decimal
+places. These are representative locality points, not postal-area centroids,
+address coordinates, post-office coordinates, or measured parcel locations.
+Decimal precision does not establish spatial accuracy.
+
+## Checks and accuracy limits
+
+Czech Post identifies [Praha 1 (11000)](https://www.postaonline.cz/detail-pobocky/-/pobocky/detail/11000),
+[Mariánské Lázně 1 (35301)](https://www.postaonline.cz/detail-pobocky/-/pobocky/detail/35301),
+and the eastern rural/border location [Mosty u Jablunkova (73998)](https://www.postaonline.cz/detail-pobocky/-/pobocky/detail/73998).
+These primary-source checks confirm postcode/locality associations. They do not
+independently validate a postcode-wide mean coordinate.
+
+The corresponding output points are respectively `[14.425633, 50.084633]`,
+`[12.710055, 49.976003]`, and `[18.793250, 49.523850]`. Mariánské Lázně includes
+29 distinct points; Mosty includes two. A rural shared postcode can cover several
+settlements, so an average can be kilometres from an actual address. GeoNames can
+estimate coordinates from nearby postcodes when a place cannot be matched. There
+is no measured national error bound. Use only for approximate distance analysis;
+these data do not establish which physical scan site or address a parcel used.
+
+## Reproduction and offline tests
+
+Extract `CZ.txt` from the linked archive, then run:
+
+```sh
+node scripts/generate-geocode-cze.js /path/to/CZ.txt > geocode/CZE.json
+npx prettier --write geocode/CZE.json
+```
+
+The generator validates source country, format, and finite coordinates within a
+Czechia bounding box. Offline tests cover every output lookup, unique keys,
+coordinate ranges, supported inputs, unknown/malformed codes, legacy DEU behavior,
+and duplicate-independent generation. No region mappings are added.