A selection deep in a long line costs a render what one at its start does: 5 MB in, /screen goes from 14.7 s to 5 ms

Every render measured a selection's and the cursor's columns from the
start of their line (lineDisplayOffset walked the whole prefix twice),
so a selection near the end of a 5 MB line stalled each frame, 3 s at
1 MB in and 14.7 s at 5 MB. The cost grew with the offset, with Wrap on
or off. Three changes make it independent of where the selection is:
- display width adds (a tab is tab_width wherever it stands), so the
  offset between two columns is the width of the text between them,
  never the prefix;
- the selection's rows are clamped to what is shown: a row wholly
  before the selection is skipped, a start before the row's first
  column starts there, and an end past its last column ends there;
- a head or cursor whose column lies past its visible row (the last
  visible row of a line owns every column after its start) is not
  measured, since it is not drawn.
Measured on a Debug build, 5 MB line: offset 10, 1 MB and 5 MB now take
11, 6 and 5 ms per /screen. A new perf gate case, deep-sel, renders with
the selection 2 MB into a line: 83 us on Debug. The baselines were
re-recorded for the new harness.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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