Add OSC 9;4 terminal progress bar to next build
Emit an OSC 9;4 progress sequence during `next build` so supporting
terminals ConEmu, Ghostty, foot, Rio, Windows Terminal, and more) render
a taskbar/tab progress bar for the build. Terminals without support ignore
the sequence, so it is safe to emit on any TTY.
The result is a progress bar rendered while the build proceeds.
The build has no single unit of total work, so it is modeled as weighted
stages that each own a band of the 0-100% range. Counted stages
(collect-page-data, static generation) fill their band from real counts;
uncounted stages (compile, type-check) ease toward the band end over time
so the bar keeps moving, then snap on completion.
Enabled only on an interactive TTY outside CI; opt out with
NEXT_DISABLE_BUILD_PROGRESS. Cleared on completion, failure, and
interrupt (SIGINT/SIGTERM) via the existing CLI process.exit path.
