Raw useTerminalDimensions fires on every SIGWINCH tick; during a drag that's a
recompute/reflow storm across every width-sensitive component (tool bodies,
tables, status bar, banner). Add a DimensionsProvider that runs the raw hook
ONCE and feeds a single leading+trailing-debounced (40ms) signal — mirroring the
gateway's 16ms event coalescing / opencode's createLeadingTrailingSignal — that
every consumer shares via useDimensions(). They now reflow together (no tearing)
and at most once per window. Falls back to the raw hook outside a provider
(headless tests). Verified: single resizes converge clean (wide banner ⇄ compact
brand at the 102-col threshold); rapid bursts coalesce. 90 pass.
alt-glitch
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