Fix capacity overflow panic for huge --line-range offset-from-end

[koopatroopa787]

Summary

bat --line-range :-N with N near usize::MAX aborted with capacity overflow (exit 101) instead of reporting an error:

$ printf 'l1\nl2\n' | bat --no-config --paging=never --line-range ':-18446744073709551614' -
thread 'main' panicked at src/controller.rs:264:62:
capacity overflow
$ echo $?
101

Two bugs combined to cause this:

1. largest_offset_from_end() + 1 in controller.rs:262 overflowed usize when the offset was usize::MAX.
2. VecDeque::with_capacity(usize::MAX) aborted with capacity overflow for the usize::MAX - 1 case.

Fix

• Replace the plain + 1 with saturating_add(1) to prevent integer overflow.
• Replace VecDeque::with_capacity with VecDeque::new() + try_reserve, which returns an error instead of panicking when the requested capacity can't be represented.

After the fix:

$ printf 'l1\nl2\n' | bat --no-config --paging=never --line-range ':-18446744073709551614' -
[bat error]: --line-range offset from end (18446744073709551614) is too large to buffer
$ echo $?
1

Normal :-N ranges continue to work unchanged.

Fixes #3845

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