wn0 batch preview-thumbnail rendering per page

Each thumbnail on a preview page (page-N.html) used to be rendered by its
own "template preview" call, paying the full source_template_file cost --
an "env -i bash" invocation -- per thumbnail. With MAXPREVIEWS thumbnails
per page that was N template renders per page just for the grid.

render_full_preview_page now builds the markup for ALL of a page's
thumbnails in bash (build_preview_thumbnail / append_preview_thumbnail)
and emits the whole grid in ONE render via a new previewpage.tmpl that
takes the pre-built HTML through a context_raw "preview_thumbs" field --
the same pattern the stats filter galleries (camera.tmpl) already use.
Per-thumbnail markup is byte-identical to the old preview.tmpl output:
same <a name=... href=...><img class='thumb <anim>' .../></a> structure,
order, HTML escaping and seeded "slow" animation class. Header and footer
stay as their own template calls, so a page now costs ~1 previewpage
render + header/footer instead of N + chrome.

The parallel job-pool integration and failure contract are unchanged:
each preview page is still one background render job. Added the
render_preview_thumbs_html field spec (hn0 dispatch pattern, context_raw
kind), registered previewpage in the validate_template_dir required
templates and in the required-context-vars test expectations, and pointed
the four generation template-failure tests at previewpage.tmpl (generation
no longer renders preview.tmpl). The standalone "template preview" engine
unit tests keep exercising preview.tmpl, which still ships.

Verified byte-identical output: generated the fixture album (including a
spaces/special-char filename) twice with RANDOM_SEED=42 using the parent
commit's bin/shuriken vs the new bin; every .html file is identical.
just test, just shellcheck, just check-generated and git diff --check all
pass.

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

1 files changed, 14 insertions, 0 deletions

diff --git a/share/templates/default/previewpage.tmpl b/share/templates/default/previewpage.tmpl
new file mode 100644
index 0000000..9f42f84
--- /dev/null
+++ b/share/templates/default/previewpage.tmpl
@@ -0,0 +1,14 @@
+# Preview-page thumbnail grid (task wn0). The album preview page (page-N.html)
+# used to render EACH thumbnail with its own "template preview" call, paying the
+# full source_template_file cost (an "env -i bash" invocation) per thumbnail.
+# render_full_preview_page now pre-builds the markup for ALL of a page's
+# thumbnails in bash (render_preview_thumbnail / build into one string) and hands
+# the whole grid in through the raw context field render_preview_thumbs_html, so
+# this template emits every thumbnail for the page in ONE render call instead of
+# N. The per-thumbnail markup is byte-identical to the old preview.tmpl output
+# (same <a name=... href=...><img class='thumb <anim>' .../></a> structure, order
+# and seeded animation classes). The surrounding header/footer still come from
+# their own templates, which render_full_preview_page emits around this one.
+cat <<END
+${render_preview_thumbs_html}
+END