bn0 guard preview_num arithmetic against non-numeric input

The preview_num next/prev render handlers computed neighbour page numbers
with $(( context_value +/- 1 )) but only guarded against an empty value.
A non-numeric preview_num context value (e.g. a stray string) slipped past
the [ -n ] check and triggered a bash arithmetic syntax error which, under
set -e, aborted the whole script.

Validate the context value is a non-negative integer
([[ value =~ ^[0-9]+$ ]]) before the arithmetic in both
prepare_template_render_var__preview_num_next_html and __preview_num_prev_html.
Invalid or missing values now default to an empty render value, matching the
existing missing-neighbour behaviour; the valid-numeric path is unchanged.

Add test_template_render_var_preview_num_guards_non_numeric in tests/cli.sh,
which drives the handlers directly under bash -euo pipefail to prove a bad
preview_num no longer crashes and a numeric one still yields the exact
+1 / -1 neighbour.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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