fix: keep CLI status output usable under ASCII locales

[abhiksark]

Closes #73.

verify and list crashed with UnicodeEncodeError on any console whose encoding cannot represent their status glyphs. Status symbols now fall back to one-column ASCII stand-ins when the stream cannot encode them, and keep the Unicode glyphs when it can:

state	UTF-8	fallback
pass	✓	+
fail	✗	x
current	●	>
locked	🔒	-

The issue's recorded scope was narrower than the bug. It names verify and list, but solution and run crash on a different path — they write the check file's captured output, and all 292 checks end with print("<name> ✓"). That glyph comes from the curriculum, not from our formatting, so degrading the CLI's own symbols would not have fixed those commands. All captured subprocess output now goes through a writer that replaces unencodable characters instead of raising.

Found while packaging for conda-forge: the win-64 test runs in a pipe using cp1252, and pythonlings solution variables1 died there.

Verification

python -m pytest -q                                          212 passed
pythonlings --root tests/fixtures/passing_curriculum verify  exit 0

Both new integration tests fail against the unpatched cli.py and pass with it. They pin PYTHONIOENCODING explicitly so the regression is deterministic rather than locale-dependent, and cover the UTF-8 path as well as the fallback.

$ LC_ALL=C PYTHONIOENCODING=ascii pythonlings --root tests/fixtures/tiny_curriculum list exercises
    >  passing
    -  asserts
    -  syntax
    -  pending

Exit codes and command semantics are unchanged; the TUI is untouched.

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