Add a shared spectrum-analyzer plot component

[vnxdtzip]

Proposed Changes

Every "signal strength across a band" screen in Bruce draws its own bars,
picks its own colours and hardcodes its own geometry. No two look alike, and
none of them scale past the panel the original author happened to own.

This adds SpectrumPlot (core/spectrum_plot), which owns the whole
presentation for that shape of data:

• layout derived from tftWidth/tftHeight, degrading on short screens by
  dropping the ruler first, then the status line, then the waterfall
• filled trace with a peak-hold line, an animated noise floor and a dashed
  reference grid visible only through the empty sky
• scrolling waterfall backed by a ring of rendered rows
• labelled ruler and a status line
• every colour derived from the active theme through blendColors

Callers keep their own radio and fill an envelope of width() values in 0-100;
the component owns every pixel. Each pixel of the band is written exactly once
per frame, so the animation is flicker free without needing a full-screen
sprite.

WifiSpectrumView (modules/wifi/wifi_spectrum) sits on top and maps the 2.4GHz
channels onto a real frequency axis as overlapping spectral lobes, so the
22MHz overlap between neighbours becomes visible and a sweep reads as one
continuous trace. It also eases the drawn levels toward the measured ones, so
the trace glides rather than snapping channel by channel.

Nothing uses either component in this PR; the modules move over separately.

Types of Changes

New Feature (internal UI component). No behaviour change and no breaking
change on its own.

Verification

Nothing on screen changes from this PR alone. The component becomes visible
through ui/channel-analyzer-spectrum, ui/jam-detect-spectrum and
ui/nrf-spectrum, which is where the rendering should be judged.

pio run -e m5stack-cardputer
git show --stat   # 4 new files, 445 insertions(+), 0 deletions(-)

Testing

No unit test harness exists for this layer. Both files were compiled for
m5stack-cardputer as part of ui/channel-analyzer-spectrum, which stacks
directly on this branch and builds clean; this branch was not built in
isolation. Rendering has not yet been checked on hardware by me - a reviewer
with a device should confirm the layout on a non-240x135 panel in particular,
since the degradation path for short screens is the least exercised part.

Linked Issues

None. Depends on #2754 (ui/color-helpers) for blendColors and buildHeatPalette.

User-Facing Change

NONE

Further Comments

Memory cost is one calloc of wfRows * plotW bytes for the waterfall history
(about 5KB on a 240px panel) plus two envelope buffers of plotW bytes each,
all freed on exit. If the waterfall allocation fails the component degrades to
a plot without history rather than refusing to start.

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Stacked PR. This branch is built on #2754. GitHub can only base a cross-fork PR on an upstream branch, so until it is merged the diff above also contains its commit. Review only the commit titled Add a shared spectrum-analyzer plot component; the diff shrinks on its own once the base lands.