KetraTermappendToPreviousCluster
Appends one grapheme-continuation codepoint to the most recently written printable cell without moving the cursor.
Parser layers call this when a cluster prefix was already published for live rendering and a later byte extends that same grapheme. Core owns the remembered printable-cell target, cluster storage mutation, wide spacer invariants, and cursor preservation.
Parameters
codepoint
Unicode codepoint to append to the previous grapheme.
Throws
if codepoint is outside 0..0x10FFFF or a surrogate, even when there is no previous cell. Invalid input never mutates state.