Fourth Phase 2 site migration. Extracts a shared _batch_download_to
helper that owns the async loop with a per-batch in_flight set, then
makes both _batch_download (the dialog-driven entry) and
_batch_download_posts (the multi-select entry) thin wrappers that
delegate to it.
Fixes the latent v0.2.3 bug where batch downloads landing inside
saved_dir() never wrote library_meta rows — _on_batch_done painted
saved-dots from disk but the search index stayed empty. The
library_meta write is now automatic via save_post_file's
is_relative_to(saved_dir()) check, so any batch into a library folder
gets indexed for free.
Also picks up filename templates and sequential collision suffixes
across batch downloads — collision-prone templates like %artist% on a
page of same-artist posts now produce someartist.jpg, someartist_1.jpg,
someartist_2.jpg instead of clobbering.
pax
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