Make the focus strip resolution-flexible (fix 176x176 layout)

[brickbots]

Problem

The focus-quality (HFD) focus strip is not fully resolution-independent. It was added in #449 before the resolution-flexible UI work in #453. #453 converted the camera preview's image resize and star selectors to derive from the display resolution, but never touched draw_focus_strip(), which kept 128-only pixel literals:

strip_top = 90        # bottom-band top edge
plot_top  = strip_top + 9
plot_bottom = res_y - 10
# bottom "det N" label at res_y - 9

On the 176×176 SSD1333 panel, strip_top = 90 lands mid-screen (~51% down) instead of as a bottom band: the exposure / matched-star labels are stranded high up with a large empty gap above the readout, and the bottom detected-star label clips at the screen edge.

Before / after (headless render)

128 (unchanged)	176 before	176 after
clean bottom band	labels stranded mid-screen, big gap, det clipped	proper proportional bottom band

On 176 before the fix, the dim band started ~halfway up the screen with 0.4s/★ labels floating far above the big readout. After the fix it's a tidy bottom band matching the 128 proportions.

Fix

Derive the geometry instead of hardcoding it (per ADR 0009):

• strip_top — a fixed fraction of screen height (res_y - round(res_y * 38 / 128)), so the band stays ~30% of the screen on any panel.
• label-row clearances — derived from self.fonts.small.height, so the rows track the larger small font on the 176 panel and never collide with the V-curve.

Because the 128 small font is 9 px, strip_top + 9 == strip_top + small_h, res_y - 10 == res_y - small_h - 1, and res_y - 9 == res_y - small_h — the 128 layout is reproduced pixel-for-pixel (zero regression) while the 176 layout becomes a correct proportional bottom band.

Verification

• Rendered the Focus screen headless on both DisplayHeadless (128×128) and DisplayHeadless176 (176×176) and confirmed the strip is a clean bottom band on each, with no stranded labels or clipping.
• nox -s lint / format / type_hints (158 files) all pass.
• nox -s smoke_tests (5) and nox -s unit_tests (280, incl. test_focus.py) all pass.

Docs

Updates the docs/ax/ui/CONTEXT.md focus strip glossary entry, which described a fixed "~38 px" band, to note it scales with the display.

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