motion, scrolling and redraw are flat in file size now

zig build perf drives the core directly — event, effects, one frame, no pty —
over four generated fixtures: 1k lines, 50k, 300k, and 400 lines of 8000
columns, because a file that is long and a file that is wide fail differently.
Every sample seeks somewhere else in the file first, since measuring at line 3
of a 300k-line file hides exactly the bug.

perf record said half the run was scanning for newlines from byte 0. So File
carries a line index, built on demand and invalidated in exactly ONE place —
setContent, the funnel every content swap already goes through. That killed the
scrollbar's per-frame line count (12.6% of the whole run by itself), scrollBy,
ensureCursorVisible, lastNavRow, the syntax window bounds and two O(scroll)
walks. normalKey computed max_line as a const at the top: two full passes over
the buffer on every keystroke of every kind, for three g/G branches. It is lazy
now. The modal primitives each walked the text twice for the same line.

And the visible window was re-parsed on every scrolled row — a third of a
megabyte per keypress on the wide fixture. The highlighted range is remembered,
a scroll inside it is free, and only a re-parse that FOLLOWS a scroll takes
slack: doing it unconditionally made typing 2.1x slower, since every character
paid for a band it could never amortise.

One j on a 19 MB file: 37.8ms -> 266us. Render: 4.1ms -> 77us. Open costs 1.25x
more for the one extra pass, which buys 54x on every frame after, and 8 bytes
per line of memory.

Left standing, measured and named: edit-char is 14ms on 19MB because content is
immutable and every keystroke copies the buffer. A third of that is the index
rebuild, which could be a shift if setContent knew the edit offset; the rest
wants a rope. bodyText's double copy and Surface.print's per-cell decode never
rose above 2% of the profile afterwards, so they were left alone.

No golden moved.

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