New --figure-folders / figure_folders option drives LaTeX's \graphicspath
in the default (non-copy) build mode. Figures that live under any of the
configured folders get short URLs in the .tex; figures elsewhere keep the
relpath-from-build-dir fallback. A single \graphicspath block is emitted
at the top of the body with each entry expressed relative to the build
dir (so the .tex stays portable as long as build/source move together).

Bundle subdir renamed build/images/ -> build/figures/ to match the new
--figure-folders / figure_folders / copy_figures terminology. The
copy-figures stale-cleanup now uses a tiny .texmark-figures manifest
inside the bundle, so any extension is handled and hand-managed files
added between builds are preserved (a previous-only allowlist of common
extensions is no longer required).

Also fixes a latent build_tex bug where filter args leaked from the
JSON-producing pandoc pass into the JSON->LaTeX pass, silently re-running
every filter twice.