New default: figures stay in place on disk; the .tex references them via
paths relative to the build dir. No more copying the entire images/ tree
on every build.

New --copy-figures (yaml: copy_figures: true) opts into a self-contained
build: figures are auto-discovered from the markdown, copied flat into
<build>/images/ with content-hash dedup on basename collisions, and stale
files from previous builds are cleaned up.

Other changes since v0.6.4:
  - SectionFilter gains drop_sections (with warning) and records
    section titles in parallel <meta_key>titles metalist.
  - apply_figure_defaults supports per-figure figure-span overrides via
    the image attribute syntax: ![cap](img){figure-span=full}.
  - --images flag is deprecated (accepted-and-ignored with a stderr
    warning); figures are now auto-detected from the markdown.