Only cut oversized segments at real gaps between items

The width-budget crop snapped to the nearest blank column, and in rendered text
the gap between two characters is a single column. So a cut routinely landed
inside a word: the cycle showed "Wednesda" and the orphaned "y" turned up as a
lone floating letter in the next cycle, positioned after whatever plugin
happened to precede it.

Measured on the clock-simple segment to confirm: its blank runs are
[1, 1, 1, 1, 1, 8, 8] — five single-column letter gaps, every one of which
find_blank_cut would happily have chosen.

Cuts now only land in a run of at least min_cut_gap blank columns (default 6),
which excludes letter spacing while still finding the gaps plugins put between
items (the stocks ticker uses 32px, baseball 48px). Where no boundary falls
inside the budget the cut waits for the next one and overruns, because
splitting an item is worse than a slightly long segment.

Continuous content is treated differently on purpose: an image with no internal
gaps is a map or a chart, where any column is as good as another, so it is still
cut to the budget exactly. The gap rule protects discrete items; letting a solid
image escape the cap in its name would be wrong.

blank_runs() is vectorised — 48ms for a 17,000px strip, against seconds for a
per-column Python loop.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

config/config.template.json

@@ -134,6 +134,7 @@
             "intra_plugin_gap": 8,
             "render_width_pct": 100,
             "min_content_separation": 24,
+            "min_cut_gap": 6,
             "auto_trim": true,
             "trim_threshold": 10,
             "content_padding": 8,