`computeCenters(double lowMhz, double highMhz, int segmentKhz)` is the
pure-math kernel that drives the ATU pre-tune sweep — it generates the
evenly-spaced tune centers for each selected band.  It had no test
coverage; the IARU R1 per-band point counts from issue #2624 (160m=23,
80m=34, 60m=2, 40m=8, 30m=2, 20m=7, 17m=2, 15m=6, 12m=2, 10m=23,
6m=20 — 129 total) were verified by hand at PR-review time and would
silently regress on a rounding/clamping change.

Lift `computeCenters` out of `AtuPreTuneDialog.cpp`'s anonymous
namespace into a header-only free function (`AtuPreTuneCenters.h`) so
the test can include it without dragging in the dialog.
`AtuPreTuneDialog.cpp` keeps using it via the new include — same
implementation, same behavior, but now reachable from tests.

`tests/atu_pretune_centers_test.cpp` exercises:
  * All 11 IARU R1 reference points (band, range, segment, expected
    point count).
  * First-center spacing (`low + seg/2`) — guards the half-segment
    offset.
  * Last-segment clamp (`high - seg/2`) — guards the "extra center if
    room at the top" path that produces #2624's expected 60m count.
  * Edge cases: zero range, inverted range, segmentKhz ≤ 0,
    sub-segment range (returns empty), exactly-one-segment range
    (returns 1, no duplicate clamp).

Registered as `atu_pretune_centers_test` in CMakeLists.txt next to the
existing `*_test` executables; runs via `ctest -R atu_pretune`.  20
assertions; passes locally.

Closes #2648.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>