Add coverage for start/end extending, snapping, never-clipping, and the
empty-plan case. The bundled sample now demonstrates the keys, so its
expected week range moves to the authored bounds.
Optional top-level `start`/`end` dates widen the plan's week range with
lead-in / trailing columns. They only extend the auto-fitted range, never
narrowing it or clipping a Feature, so a momentarily-tight bound can't hide
work in progress.
Captures the full timeline (not just the scrolled viewport) by expanding
the grid to content width during render; clipboard write stays inside the
user gesture so Safari accepts it, with download as the documented fallback.
- milestones render as horizontal label flags, auto-stacked onto extra
rows with leader lines so labels in nearby weeks never collide
- "today" becomes a neutral grey column (mixed in oklab) and milestone
diamonds are grey too — red is reserved for problems (late, red status,
unmet counts), which no longer get a stray warm tint
- fix the name column to a stable width so the band aligns with the grid
- bar line begins/ends at a glyph centre — no stub past the first/last symbol
- glyph halos so the bar passes behind, not through, the symbols
- today's week is a shaded column instead of a vertical rule
- widen week columns so date labels stop colliding
- drop "slip" wording from the legend
