fix: normalize micro codepoint to U+00B5 in Units

[bluetoothbot]

What: Normalize the two "micro" units that used the Greek letter mu (U+03BC) to the micro sign (U+00B5).

Why: TIME_MICROSECONDS (µs) and CONCENTRATION_MICROGRAMS_PER_CUBIC_FOOT (µg/ft³) were declared with U+03BC, while every other micro unit in the enum — and the Home Assistant constants this library mirrors — use U+00B5. The glyphs render identically but compare unequal, so string equality against HA's unit constants silently failed for exactly these two members. Closes #143.

How: Two-character value change in units.py; member names and all other values are untouched. Added tests/test_units.py pinning every micro unit to U+00B5 so the inconsistency can't recur.

Testing: New regression tests fail on main and pass on this branch; full suite (10 tests) green under Python 3.12, ruff clean.

---

Quality Report

Changes: 2 files changed, 27 insertions(+), 2 deletions(-)

Code scan: clean

Tests: passed (10 passed)

Branch hygiene: clean

Generated by Kōan

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.37%. Comparing base (31bd826) to head (a5c3097).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #145   +/-   ##
=======================================
  Coverage   94.37%   94.37%           
=======================================
  Files          12       12           
  Lines         480      480           
  Branches        5        5           
=======================================
  Hits          453      453           
  Misses         26       26           
  Partials        1        1           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.