v0.3: Sefaria importer

[bsesic]

Closes #11. Foundation for #12.

Adds `tracealign.io.sefaria` with three public functions, exercised by offline-fixture tests for Davidson, Vilna, and Kaufmann editions of Mishna Avot 1.

Public surface (`tracealign.io.sefaria`)

• `load(ref, version, lang, seq_label)` — fetch one Sefaria reference, concatenate segments, return `list[Token]`.
• `load_segments(ref, version, lang, seq_label)` — return one token list per segment (e.g. per mishna), with derived `seq_label`s of the form `{base}:{n}`.
• `load_versions(ref, versions, lang)` — convenience for multi-witness; `dict[version_title, list[list[Token]]]`.

Implementation notes

• HTTP via stdlib `urllib.request` — no new runtime dependencies.
• Module-level `_http_get` is the patch target for tests; `_fetch_json` wraps it with UTF-8 decoding and `json.loads`.
• `_strip_html` removes `//...` tags and drops `<sup>...</sup>` footnote markers entirely so footnote digits never enter the token stream.

Stats

• 10 new tests covering fetch, load, load_segments, HTML stripping, multi-version, and the module-surface pin.
• Suite at 192 passing, `flake8` clean, Sphinx -W clean.
• Three fixture JSON files exercise EXACT / NIQQUD_STRIPPED / PLENE_DEFECTIVE tiers when pairwise-aligned.

Test plan

• CI matrix passes on Python 3.10 / 3.11 / 3.12
• `pytest -q` locally → 192 passed
• `flake8 src/ tests/` → no output
• `sphinx-build -W -b html docs docs/_build/html` → build succeeded
• Manual: live-fetch a Mishna from Sefaria once 0.3.0 ships