fix(core,engine): auto-hide the watermark in cramped containers

Charts compiled at a cramped container height (< 200px) now suppress the
default watermark, mirroring the sparkline suppression and keyed on the
same userExplicit gate: an explicit watermark at top level or in the
active breakpoint override still wins. At those heights chromeMode is
already 'hidden', so the brand previously painted over the plot's bottom
edge in short-but-wide dashboard tiles.

computeChrome's hidden branch now reserves the brand band whenever the
watermark survives to it (explicit opt-in at cramped heights, or the
chromeEatsPlot fallback at normal heights) instead of returning zero and
letting the brand overlap the plot, and surfaces a bottom-legend
reservation the way the compact branch does.

Schema and llms artifacts regenerated for the watermark JSDoc change.

Claude-Session: https://claude.ai/code/session_0172Y6ogNcx8KvZR2pCEo6Ho