The pre-built `1..=parse_last_page` queue silently broke whenever the
configured CRAWLER_START_URL lacked a `/N/` path segment: page_url
returned the input unchanged, every "next" page re-fetched page 1, and
the dedup set caught the duplicates as a flood of "skip already-seen
key in this run" debug lines. The walker now increments next_page on
each batch and terminates when parse_manga_list_from yields an empty
list (the `#logo` sentinel still converts unrendered pages into
transient errors, so an Ok(vec![]) is a real end-of-index signal).
parse_last_page and build_page_order are deleted along with their
unit tests; they have no callers under the new model. page_url and
the page-1 HTML cache from discover() are retained as-is.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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