feat: unify image stats and use ihist

[tlambert03]

this consolidates two image statistics paths that used to be serial and independent (calculation of clims and the conditional calculation of histograms) into a single compute_image_stats, backed by ihist (thanks @marktsuchida !)

Scenario	Old (ms)	New (ms)	Speedup	Strategy
uint8, hist+minmax	5.35	0.33	16x	ihist → minmax from counts
uint16, hist+percentile	35.44	1.00	35x	ihist → percentile from CDF
uint16 12-bit, hist+minmax	5.41	0.46	12x	ihist(bits=12) → minmax from counts
float32, hist+minmax	60.28	19.71	3.1x	nanmin/max → np.histogram(256)
uint16, no hist, percentile	28.06	1.00	28x	ihist → percentile from CDF
float32, no hist, minmax	0.34	0.33	1x	nanmin/nanmax (unchanged)
float32, no hist, stddev	8.73	8.73	1x	nanmean/nanstd (unchanged)

[codecov]

Codecov Report

❌ Patch coverage is 95.08197% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.96%. Comparing base (41e6da6) to head (b0358dc).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ndv/controllers/_image_stats.py	94.50%	5 Missing ⚠️
src/ndv/controllers/_channel_controller.py	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #249      +/-   ##
==========================================
+ Coverage   86.85%   86.96%   +0.11%     
==========================================
  Files          46       47       +1     
  Lines        5347     5441      +94     
==========================================
+ Hits         4644     4732      +88     
- Misses        703      709       +6     

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