docs: document public usage (manager, wrappers, active scan)

[bluetoothbot]

What: Replace the TODO: Document usage stub in docs/usage.md with a real, code-grounded usage guide.

Why: The published docs (readthedocs) had no usage content for a library with a 38-symbol public API. Anyone landing on the Usage page got import habluetooth and a TODO. This fills the gap without touching code.

How: Documents the actual public surface, every claim verified against the source:

• the BluetoothManager singleton + get_manager/set_manager/async_setup/async_stop lifecycle
• HaBleakScannerWrapper / HaBleakClientWrapper as drop-in bleak replacements
• history queries (async_last_service_info, async_discovered_service_info, async_ble_device_from_address, …)
• the on-demand active-scan API (async_register_active_scan / async_request_active_scan), including the clamp/coalesce semantics from their docstrings
• the storage round-trip helpers and the rebuildable-cache rationale

Testing: prettier --check and codespell pass on the file. Docs-only change (docs: type, excluded from the changelog); no source touched, full test suite unaffected.

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Quality Report

Changes: 1 file changed, 181 insertions(+), 1 deletion(-)

Code scan: 2 issue(s) found

• docs/usage.md:72 — debug print statement
• docs/usage.md:113 — debug print statement

Tests: failed (FAILED)

Branch hygiene: clean

Generated by Kōan

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (ac56ebe) to head (cfd643a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #538   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files          15       15           
  Lines        2689     2689           
  Branches      386      386           
=======================================
  Hits         2678     2678           
  Misses          1        1           
  Partials       10       10           

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[codspeed-hq]

Merging this PR will not alter performance

✅ 26 untouched benchmarks
⏩ 5 skipped benchmarks¹

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_{Comparing bluetoothbot:koan/docs-usage-guide (cfd643a) with main (ac56ebe)}

Footnotes

1. 5 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩