fix: make the GUI helpers' colour scale constructible through the public API

[Jammy2211]

Closes part of #585.

Cmap is no longer exported by any public plot namespace — not
autoarray.plot, autogalaxy.plot, autolens.plot or autocti.plot. Two GUI
helpers in this repo still required one, so their colour scale could only be
supplied via a private-path import. A third gap surfaced while migrating the
downstream caller.

API Changes

Scribbler.__init__ — new arguments, backward compatible.

# before — needed an object no public namespace exports
al.Scribbler(image=..., cmap=aplt.Cmap(cmap="jet", norm="log", vmin=1e-3, vmax=3.0))

# after
al.Scribbler(image=..., cmap="jet", norm="log", vmin=1e-3, vmax=3.0)

norm, vmin and vmax are added and cmap now accepts a colormap name. A
legacy Cmap-style object exposing norm_from is still accepted and takes
precedence, so any caller holding an instance keeps working. Passing nothing is
unchanged.

plot_array — new optional mask= argument.

aplt.plot_array(array=data, mask=mask, positions=centres, use_log10=True, ...)

The wrapper delegates to autoarray.plot.plot_array but was silently dropping
that function's mask= parameter. Because the outline is otherwise auto-derived
from array.mask, a caller wanting to outline a different mask — the ordinary
preprocessing case of showing a candidate mask radius over unmasked data — had
no way to ask for it. Purely additive; omitting it preserves current behaviour.

Clicker.start() — bug fix, no signature change. It built
aplt.Cmap(...) on autoarray.plot unconditionally, so it raised for every
caller:

AttributeError: module 'autoarray.plot' has no attribute 'Cmap'

Removed: nothing. Deprecated: nothing.

Changes

• util/plot_utils.py — adds norm_from(array, use_log10, vmin, vmax),
  mirroring the normalisation autoarray.plot.array.plot_array applies so code
  drawing its own axes scales colour identically; adds _mask_edge and the
  mask= passthrough.
• gui/scribbler.py — flat colour arguments, legacy object path retained.
• gui/clicker.py — uses the helper; its hardcoded scale is unchanged.
• __init__.py — drops a duplicate Scribbler import present at both line 76
  and line 124.

Testing

test_autogalaxy/gui/ is new (the directory did not exist). 16 tests added
across two files: norm construction, the new signature, _mask_edge's contract,
and a regression guard asserting Cmap stays absent from the public namespaces.

The mask passthrough is verified by rendering, not just binding: on an
unmasked array auto_mask_edge returns None and no outline appears, while
passing mask= changes 2436 pixels of the output PNG.

Full suite green locally: 1129 passed on Python 3.12. CI covers 3.13.

Limitations, stated plainly

• Scribbler.__init__ has no direct test. It calls matplotlib.use("TkAgg")
  and mng.window.wm_geometry, so it cannot run headless. The extracted
  norm_from helper carries the logic and is fully tested; the constructor's
  branching is covered only by signature binding.
• Only Python 3.12 was run locally. Nothing here is version-sensitive, but
  3.13 is unverified until CI runs.

Follow-up, not fixed here

autoarray duplicates this same normalisation inline in plot/array.py and
plot/inversion.py. A shared helper there would be the real fix; this adds one
copy in autogalaxy serving both GUIs rather than a third inline copy.

Downstream

euclid_strong_lens_modeling_pipeline has a companion branch of the same name
that depends on both API changes. It must merge after this PR, per the
library-first gate.

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