A view URL without a page-id crashed builds with assertions enabled:
the viewer passes a nil page-id to `initialize`, whose schema accepted
a missing key but not a nil value. Release builds skip that check and
draw the first page, but every viewer event reads the page from the
route, so zoom to fit and fill, the next-board arrow, Navigate To
interactions, and comment pins did not work.
`initialize` now accepts a nil page-id, `fetch-bundle` makes it
optional, and `bundle-fetched` replaces a route without page-id by one
naming the file's first page. The viewer then initializes again on the
completed route, as it does when the user changes page. A file with no
pages keeps its route.
AI-assisted-by: claude-opus-5-5
Signed-off-by: Álvaro Tejero Cantero <alvorithm@teje.ro>
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