test: add narrative/solar-arc tests and CI workflow

[kochj23]

Summary

sunlog had no tests and no CI. This PR adds both:

• tests/test_sunlog.py — exercises the daily-narrative / solar-arc logic directly by feeding synthetic readings into DayTracker. No network and no real Powerwall are needed; only urllib is mocked, and that only for the two fetch() resilience tests. Coverage includes:
  • parse() typed CSV parsing (and coercion of hostile/malformed remote values)
  • solar-arc tracking: online / peak / fade timestamps and battery-full detection
  • _kwh() sign-filtered energy estimation and _pct_solar() fraction
  • the CSV round-trip that lets sunlog survive a mid-day restart
  • the English narrative() output: online/peak/fade times, battery delta ("up/down N% from …"), grid import/export lines, and one-line story selection
• .github/workflows/ci.yml — runs pytest on Python 3.9–3.13.

The suite is stdlib + pytest only, matching the project's dependency-light style.

Test categories covered

1. Security — fetch() swallows network errors and returns None (a failed poll never crashes the daemon); parse() coerces injected/non-numeric CSV field values to 0.0 instead of raising.
2. Performance — save_raw_csv() appends only new readings; a call with nothing new neither rewrites nor duplicates on-disk rows.
3. Retry — N/A: fetch() makes a single request and returns None on failure; recovery is the next scheduled poll, not in-call retry/backoff. Marked with an explanatory @pytest.mark.skip placeholder rather than omitted.
4. Unit — parse() typing, _pct_solar() fraction, _kwh() sign filtering, and ingest() arc/battery-full tracking.
5. Integration — CSV written by save_raw_csv() reloads via load_from_csv() into a fresh tracker that reproduces the same arc and identical narrative; save_narrative() writes the summary file.
6. Functional — a full sunny day yields a narrative naming the online/peak/fade times, battery delta and export; a 100%-solar day selects the "nearly perfect solar day" story.
7. Frame — boundary cases: no data at all, a no-sun day (falling battery reads "down", "No sun today" story), and _kwh() returning 0.0 with fewer than two samples.

Verification

pytest tests/ → 14 passed, 1 skipped (the Retry N/A placeholder).

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