Migrate to released Mojo 1.0 and cut 0.2.0

[conorbronsdon]

Mojo 1.0 shipped, so this comes off nightly. pixi moves to the max channel with mojo >=1.0,<2, and CI installs mojo==1.0.0 from PyPI instead of the nightly index with --prerelease allow. The weekly drift check still points at the latest nightly, so the repo keeps telling us when the language moves under it.

No source changes were needed — the parser compiles unchanged on 1.0.

The README also gets its numbers redone. The test count was stale (70 → 102 — the errors module added a fifth test file), the corpus paragraph now describes what the live run actually returns, and the throughput range is restated for the machine it was re-measured on.

Cuts 0.2.0.

Verified locally on Mojo 1.0.0: 102 tests green across all five files, mojo format clean, benchmark builds and runs.

[conorbronsdon]

Verification notes (generated — detail for reviewers and review tooling)

All of the below ran locally on Mojo 1.0.0 (ed45d567), Ubuntu on WSL2, against this branch.

• Tests: test_xml_parser 32, test_feed 23, test_errors 26, test_date 11, test_real_feed 10 — 102 total, 0 failed.
• Fuzz (test/fuzz_drive.py, 5,400 seeded mutations): 0 crashes, 0 hangs.
• Corpus (test/corpus_run.py, live 144-URL OPML): 137 fetched; 0 raised, 0 crashed, 0 hung; 133 clean, 3 with a literal <title></title> in source, 1 (graydon2.dreamwidth.org) served a 504 HTML error page rather than a feed — confirmed by re-fetching it directly.
• Throughput: the README said 131–146 MB/s; this machine measures 113–135 MB/s. That is a machine difference, not a 1.0 regression — b3 and 1.0 were built from the same tree and run back to back in two interleaved rounds on this box (b3 128.99 / 134.80 / 115.92 MB/s vs 1.0 127.98 / 135.43 / 113.21 MB/s).