Add the Python variant, agreeing with Go byte for byte

A second, independent implementation of the same contract, in the same repo:

- swehockey/dom.py builds the tree on html.parser, so the variant has no
  runtime dependencies; the only HTML5 behaviour it depends on - stray <tr>
  landing in an implicit <tbody> - is handled explicitly by table_rows().
- parse.py mirrors the Go layout profiles, dual-render checks, carry-forward and
  invariants; a DriftError is raised in the same places with the same advice.
- cli.py offers the same three commands, flags and exit codes as the Go main.
- tests/ assert against the Go golden files, so the suite is also a
  cross-implementation check. 23 offline tests plus an opt-in live test.

Verified against the live site: JSON and CSV are byte-identical to the Go
output (364 games, 102536 bytes / 365 lines), and the full archive crawl gives
the same 27 seasons, 2119 games and 1779 played.

Also split the DriftError hint out in Go, so a golden mismatch no longer tells
the reader to extend layouts[].

python/README.md

@@ -0,0 +1,70 @@
+# swehockey-schedule (Python)
+
+The Python twin of the Go crawler in the parent directory. Same layout profiles,
+same invariants, same canonical order — and, on purpose, **the same bytes**.
+
+```console
+$ python3 -m swehockey.cli schedule --season 20961 --format csv | head -2
+season_id,game_id,event_id,date,time,home,away,home_goals,away_goals,periods,spectators,venue
+20961,90001001,1109941,2026-09-19,15:15,Frölunda HC,Växjö Lakers HC,5,4,1:1-1 2:2-1 3:2-2,12044,Scandinavium
+
+$ python3 -m swehockey.cli crawl --league 20961     # every season, as NDJSON
+$ python3 -m swehockey.cli leagues                  # 119 competitions
+$ python3 -m unittest discover -s tests -t .        # 23 offline tests
+$ SWEHOCKEY_LIVE=1 python3 -m unittest discover -s tests -t .   # + the site
+```
+
+With `pip install -e .` the same CLI is on `$PATH` as `swehockey-schedule`.
+
+## Why it can agree byte for byte
+
+- Same `LAYOUTS` profiles, keyed on the header row; same per-field shape
+  checks; same dual-render agreement for the modern layout; same carry-forward
+  for the date heading and the round number; same invariants (periods sum to
+  the score, ids unique, played ⇔ event link).
+- Same canonical sort by `(date, time, game_id)`, because the site orders rows
+  by round and puts re-scheduled games where their round says.
+- The output carries only page-derived data — no fetch timestamp, no unordered
+  iteration, fixed key order — and `json.dumps(..., indent=2, ensure_ascii=False)`
+  is byte-compatible with Go's `json.Encoder`.
+- The tests read the **Go golden files** in `../testdata`, so the suite doubles
+  as a cross-implementation check.
+
+Verified on the live page, not just on fixtures:
+
+```
+$ python3 -m swehockey.cli schedule --season 20961 --out py.json
+$ ../swehockey schedule        -season 20961 -out go.json
+$ cmp py.json go.json && echo identical
+identical          # 364 games, 102536 bytes
+                   # and the same for CSV: 365 identical lines
+                   # and for the whole archive: 27 seasons, 2119 games, 1779 played
+```
+
+## No runtime dependencies
+
+The tree is built by a ~120-line builder on `html.parser`, because the only part
+of the HTML5 tree construction algorithm this crawler relies on is that stray
+`<tr>` elements land in an implicit `<tbody>` — which `table_rows()` handles
+explicitly, exactly as in Go.
+
+One deliberate difference from a browser: the site writes empty cells as
+`<th class="…" />`, and HTML5 ignores that slash, so a real browser leaves the
+cell open and swallows its siblings. This builder treats the tag as an empty
+element, which is what the author meant and what keeps `child_elements()` honest.
+That is also why the Go and Python trees agree on the header row (7 `th`, 6
+non-empty labels) even though the Go parser closes the tag implicitly.
+
+## Modules
+
+| module            | what it does                                              |
+| ----------------- | --------------------------------------------------------- |
+| `dom.py`          | the HTML tree builder and the node helpers                 |
+| `parse.py`        | layout profiles, the game reader, the invariants           |
+| `seasons.py`      | league and season discovery                                |
+| `fetch.py`        | one GET per page, charset guard, skip-unchanged state file |
+| `output.py`       | JSON, NDJSON, CSV, golden-file checking                    |
+| `cli.py`          | `schedule`, `crawl`, `leagues` and the exit codes          |
+
+Exit codes match the Go CLI: `0` ok, `1` error, `2` usage, `3` markup drift or
+golden mismatch, `4` timeout.