Tier-1 comfort pass for the served trace chart (#103): an ordinal x-axis
default (collapses non-trading gaps) with a one-click continuous toggle, plus
wheel-zoom / drag-pan / dblclick-reset, a value-readout legend, and panel
cursor-sync. Confined to the read/render side (chart-viewer.js + render.rs
CHART_HEAD); the Rust serve contract is unchanged.
Boss-signed: grounding-check PASS (five current-behaviour assumptions ratified
by green node guards + render.rs inline tests). Derived-fork decision log in
the #103 comments.
refs #103
