The viewer ToggleNroff command toggled v.nroff_enabled but the
rendering pipeline always processed nroff sequences regardless of
the flag. Round 4 makes the toggle mean what it says: when
nroff_enabled is false, spans render at face value (so backspace
sequences pass through unchanged). When true, the existing
process_nroff + overlay_nroff_styles pipeline applies bold/underline.
The processing code (nroff.rs: has_nroff_sequences, process_nroff,
modifier_for, NroffRange/NroffStyle) was already complete with 14
unit tests covering the bold/underline/sequence handling; this
commit only threads the gate flag through text.rs.
Tests: 1486 passing (was 1486). No regression.
Sisyphus
d878b19ed1