slides: fix slide 1 text-anchor overflow, renumber slide 2 footer

Slide 1 bug: bullet list in the quicknxsv2 (Qt GUI) box was missing
text-anchor=middle on elements positioned at x=460. Default anchor
start caused lines to overflow the right edge of the 760px box
(visible as clipped "peak_finding", "_as_ints", "drift" text in the
earlier render).

Fix: switch to an explicit two-column left-aligned bullet layout
inside the box, so each line starts at a known x. Also improve
the "tangle" arrows between the three backend sub-boxes — replaced
the stubby Bezier arcs that rendered as tiny artifacts with
clearly-visible longer arrows plus annotations ("imports 4 files",
"15 direct sites", "parallel code") that name the couplings.

Slide 2 footer: update slide count "2 of 2" → "2 of 8" now that
six additional slides are coming.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>