fix(engine): add missing điu pattern to _consonantD table

[mantrandev]

Summary

• {KEY_D, KEY_I, KEY_U} was missing from _consonantD in Vietnamese.cpp
• In VNI mode, typing diu91 produced literal diu91 instead of đíu
• Only the reversed order d9iu1 worked because the buffer held just [d] when 9 arrived, matching the existing {KEY_D} entry

Root cause

When 9 (the VNI đ-key, IS_KEY_D) is pressed after diu, the engine scans _consonantD for a matching syllable pattern. {KEY_D, KEY_I, KEY_U} did not exist, so the engine fell through and inserted 9 as a literal character. After that, 1 (sắc) found a non-vowel-ending buffer and also fell through as a literal.

Test plan

• Type diu91 in VNI mode → should produce đíu
• Type d9iu1 in VNI mode → should still produce đíu
• Type diu + other tone marks (diu92, diu93, etc.) → verify tones apply correctly