Pin the column toggle's cursor-follow; a group lookup stops at its row (improvise-pb30, improvise-0r01)

pb30: gz puts the cursor on the collapsed group through landing and
place_cursor, and their column arms were untested. Three rivals passed
all 1948 tests: the arm swapped, landing returning None for columns, and
landing reading col_tuple(0). A new test uses two column levels
(Region over Month) with the cursor at (1, 7), South/May. It requires
the cursor to land on South's Q2 at (1, 5). Each rival fails it and
nothing else. The col_tuple(0) rival needs the two column levels, since
with one level the prefix is empty everywhere.

0r01 (Codex's P2 on #263): innermost_group built open_runs for the
whole axis, cloning a level stack per row, to answer for one row on
every z. open_runs is now a lazy iterator, still the one reader, and
the lookup takes .nth(idx). a_lookup_does_not_read_past_its_row gives
it an unreadable header after row 0 (level usize::MAX). The eager rival
panics with capacity overflow.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/code/core.md

@@ -302,7 +302,10 @@ previous survivor, pruned against unpruned. It runs over five fixtures,
 including a non-contiguous group, a level with no groups, and two adjacent
 outer groups. Without the non-contiguous fixture, a pruner that keyed runs by
 name instead of header position passed every test. `open_runs` is the one
-reading of the markers: `innermost_group` and the tests both use it.
+reading of the markers: `innermost_group` and the tests both use it. It is a
+lazy iterator, so a single lookup (`z`, which asks about one row) walks the axis
+only up to that row, rather than building and cloning a stack for every row of
+it (improvise-0r01).
 
 A header's level also names its category (`cats[level]`) on the wire, where
 `FrameGroup` carries `level` and not the category (improvise-7xyx removed that