hn0 make template render kind dispatch extensible (OCP)

Replace the hardcoded `case "$kind"` block in
prepare_template_render_vars with a name-based registration/dispatch
pattern. Each render field kind is now implemented by one
prepare_template_render_var__<kind> handler; the core loop resolves the
handler by name (prepare_template_render_var__$kind), verifies it exists
via `declare -F`, calls it with a uniform signature
(out_nameref, context_name, source_name), and reports a config_error for
an unknown kind (no matching handler) -- preserving the previous error
behavior. Adding a new kind now means defining a new handler function
only; the loop never changes.

Handlers cover all existing kinds: context_css, context_html,
context_raw, current_date_html, config_html (keeps its inner source_name
dispatch for HEIGHT/MAXPREVIEWS/TITLE/etc. and the same :- defaults),
original_basepath_is_set, preview_num_next_html, preview_num_prev_html,
and tarball_include. Escaping and defaults are unchanged, so rendered
output is byte-identical (verified by diff -r of a full --generate
album, before vs after).

Add test_template_render_var_dispatch_is_extensible proving every
declared kind resolves to a handler and that a newly defined handler is
dispatched without touching the core loop (OCP). just test, just
shellcheck, just check-generated and git diff --check all pass.

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

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