pr: fix OOM on infinite streams and optimize merge mode

[0xScodyx]

Summary

Fix OOM on infinite streams (/dev/zero, /dev/random) in both single-file and -m merge mode. Additionally optimized merge mode to be ~2x faster than GNU pr.

Changes

• Streaming: read one page at a time, print, discard — never hold more than one page in memory
• MAX_CHUNK_SIZE = 256KB: artificial newline when input has no line/FF separators
• read_one_page(track_content=false): skip pages outside --pages range without allocation
• write_merge_page(): specialized for -m mode with pre-allocated buffers and direct byte writes (no intermediate String allocations)
• BufWriter: batch stdout writes, reduced syscall overhead significantly
• Removed dead code: get_pages(), get_file_line_groups(), to_table_merged(), etc.

Benchmarks (200MB × 2 files, -m merge mode)

Version	Wall time	vs GNU
Rust (before)	1.30s	~20% faster
Rust (after)	0.79s	~2x faster
GNU pr	1.57s	baseline

Tests

• All 230 test_pr tests pass (including memory-limit test)
• /dev/zero under ulimit -v 300000 runs without OOM in both modes

Closes #13479

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/misc/io-errors (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[Devel08]

there's no such flag as --start-page=N, neither in GNU nor in uutils

[0xScodyx]

it was just a personal note in the code i forgot to remove