Function is_accent_marker 

pub const fn is_accent_marker(b: u8) -> bool

Branchless membership test against ACCENT_MARKERS.

Compiles to (b < 128) & ((MASK >> b) & 1) — one cmp, one shift, one AND — no memory load, no loop, no branch. Replaces the prior ACCENT_MARKERS.contains(&b) linear scan over 9 bytes.