feat: explore profile chart + Gaussian stats (xc/yc/fwhm/amp/bkg)

Adds cv-profile canvas and explore-stats aside to _mini(); new
explore-stats.js draws column/row projection profiles (info/warning
colors) and estimates centroid, FWHM, peak and background analytically
from the tile float32 data. Also fixes centerIndex in
_renderExploreTile (was pointing to row-start instead of tile centre).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>