kn0 compute only required template render vars per template

prepare_template_render_vars previously computed and serialized every one
of the 30+ render_* fields for each template invocation, even though e.g.
the header template references only a handful. It now computes only the
subset the target template actually needs.

The needed set is driven by the required_templates (5th) field of each
TEMPLATE_RENDER_FIELD_SPECS entry, corrected/completed so every spec's
required_templates exactly matches that render_var's references in the
.tmpl files (config_html and derived kinds previously left it empty). A
new template_needed_render_vars_to builds the per-template set;
prepare_template_render_vars takes the template name and skips non-needed
fields; serialize_template_render_context emits only the computed keys
into the BASH_ENV context file.

Dependency closure: handlers read only from the input context array or
config globals, never from another computed render_var, so the direct
per-template set is the full closure (no transitive expansion needed).
Side-effects: every handler is a pure value computation; none consume
RANDOM/seed; current_date_html only primes the deterministic
SHURIKEN_CURRENT_DATE_TEXT cache (idempotent), so subsetting is safe for
all fields. render_html_dir_html (required_templates='*') is referenced by
no template but kept always-computed as a documented cheap exception.

Output is byte-identical: diff -r over a full generated dist/ (3-image,
2-camera fixture, STATS_PAGE=yes, fixed random seed) between the previous
bin/shuriken and this build matches exactly across all 63 files (51 HTML
album/details/splash/stats/per-camera/filter pages + tarball).

Tests: add test_template_render_vars_subset_is_minimal_for_header (header
computes only its needed vars, succeeds with unrelated config globals
unset) and test_template_render_var_subsetting_matches_templates (spec
needed-set equals each .tmpl's render_* references). Existing
test_template_required_context_vars_come_from_render_specs is unchanged.

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

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