more: replace InputType enum with Input struct; cache file_size

[aaron-ang]

Motivation

The pager dispatched between BufReader<File> and Stdin via a 2-variant enum, and called metadata() every time someone asked for the file size — even though the answer never changes during the run.

Changes

• Replace enum InputType with a struct Input holding a Box<dyn BufRead> plus a cached file_size: Option<u64>, populated once at construction.
• Constructors Input::from_file (reads metadata().len()) and Input::from_stdin (size unknown).
• Input::len() now returns Option<u64> instead of Result<Option<u64>> — the only fallible call (metadata()) is gone.
• Drop the duplicate Pager::file_size field; draw_status_bar reads from self.input.len() directly.
• Pager::new no longer returns UResult<Self> — nothing inside it can fail anymore.

Drive-by

• Rename page_resize → resize_page (verb-first matches every other Pager::* method).
• Promote MULTI_FILE_TOP_PROMPT block comment to a doc comment.

Carved out from #10667 per the request to split.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)