Guard L2 reordering against the maximum implicit level (126)

[gaoflow]

BidiInfo::visual_runs panics with Level error: OutOfRangeNumber when a line
sits entirely at the maximum implicit level 126. In the L2 step,
min_level.new_lowest_ge_rtl() evaluates Level::new(126 | 1) = Level::new(127),
which is past the ceiling, so the following .expect(...) panics.

Level 126 is reachable by ordinary text: nest isolates/embeddings to the explicit
depth 125, then a digit inside is raised to implicit level 126 by rule I2 (as noted
in the issue, implicit resolution tops out at 126). Such a line is uniformly 126,
all even, so nothing needs reordering.

reorder_visual already guards this with if min == max && min.is_ltr(). The
other two L2 implementations were missing it. This adds the same guard to
visual_runs_for_line (which backs visual_runs for both utf-8 and utf-16) and
to the deprecated visual_runs. For a uniform even level the guard matches what
the existing loop already produces, so behaviour is otherwise unchanged and the
conformance suite still passes.

Repro:

let mut s = String::new();
for i in 0..140 { s.push(if i % 2 == 0 { '\u{2067}' } else { '\u{2066}' }); }
let d = s.len(); s.push('9');
let bidi = BidiInfo::new(&s, Some(Level::ltr())); // level at d == 126
bidi.visual_runs(&bidi.paragraphs[0], d..d + 1);  // panics before this change

Fixes #145.