feat: native Xiegu CAT backend (G90 / X6100 / X6200 / X5105)

Xiegu speaks CI-V with a reduced command set: frames, BCD encoding, addressing
and the opcodes for frequency, mode, PTT and split are Icom's, so this reuses
internal/cat/civ wholesale instead of re-deriving a codec.

It is a SEPARATE backend rather than the Icom one at address 0x70, because what
the two rigs do NOT share is the deciding part. The Icom backend reads the
spectrum scope, the DSP block, data mode via 1A 06 and the model id via 19 —
none of which a Xiegu implements. Pointed at a G90 it would poll every cycle for
answers that never come, and spend its silence tolerance on commands the radio
was never going to support.

Two consequences of the rig's smaller mode table are handled explicitly rather
than left to fail: there is no data mode, so a digital QSO is set to plain
sideband (what the operator does on the radio anyway) instead of being refused;
and the split TX frequency is NOT reported, because reading the unselected VFO
needs 0x25, which the Xiegu table does not list — a split flag carrying a wrong
TX frequency is worse than the flag alone, since the frequency is what gets
logged.

The published command table has rows that slipped during typesetting (0x07 and
0x0F share a block). Where it contradicts itself the Icom meaning is used, the
rest of the table matching Icom exactly, and every unexpected reply is logged
raw so a first on-air run settles it.

NOT yet verified on a radio.

changelog.json

@@ -3,6 +3,7 @@
     "version": "0.21.9",
     "date": "2026-07-28",
     "en": [
+      "Native Xiegu CAT (G90, X6100, X6200, X5105): a new backend talks CI-V straight to the radio — frequency, mode, split and PTT. Pick \"Xiegu (native CI-V)\" in Settings → CAT. Untested on a radio so far, feedback welcome.",
       "CAT sharing: OpsLog can now serve its rig connection to other programs (Settings → CAT → Share CAT). WSJT-X, JTDX, MSHV or Log4OM connect with rig model \"Hamlib NET rigctl\" at 127.0.0.1:4532 — needed because a native CAT backend holds the radio serial port on its own. Works with every backend, and with both Hamlib command dialects (JTDX names the VFO, MSHV does not).",
       "Native Yaesu CAT: a new backend talks to an FTDX10/FTDX101 directly over its serial port, without OmniRig — frequency, mode, VFO A/B, split and PTT. Pick \"Yaesu (native CAT)\" in Settings → CAT. First release, feedback welcome.",
       "Changing band from OpsLog now updates the displayed frequency straight away. The rig moved, but its answer arrived inside the short grace window that protects what you are typing and was discarded — so the frequency stayed on the old band until you nudged the VFO.",
@@ -14,6 +15,7 @@
       "CQ and ITU zones you correct by hand in your profile stay corrected. They are derived from cty.dat, which gives the zones of the whole entity — in a country spanning several, the automatic value is wrong and it came back at every restart. They now only fill in when empty, and recompute when the callsign or grid changes."
     ],
     "fr": [
+      "CAT Xiegu natif (G90, X6100, X6200, X5105) : un nouveau backend dialogue en CI-V directement avec la radio — fréquence, mode, split et PTT. À choisir dans Réglages → CAT sous « Xiegu (CI-V natif) ». Pas encore testé sur une radio, retours bienvenus.",
       "Partage du CAT : OpsLog peut désormais servir sa liaison radio aux autres logiciels (Réglages → CAT → Partager le CAT). WSJT-X, JTDX, MSHV ou Log4OM s'y connectent avec le modèle « Hamlib NET rigctl » sur 127.0.0.1:4532 — nécessaire car un backend CAT natif occupe seul le port série. Fonctionne avec tous les backends, et avec les deux dialectes Hamlib (JTDX nomme le VFO, MSHV non).",
       "CAT Yaesu natif : un nouveau backend dialogue directement avec un FTDX10/FTDX101 par son port série, sans OmniRig — fréquence, mode, VFO A/B, split et PTT. À choisir dans Réglages → CAT sous « Yaesu (CAT natif) ». Première version, retours bienvenus.",
       "Changer de bande depuis OpsLog met désormais la fréquence affichée à jour immédiatement. La radio se déplaçait bien, mais sa réponse arrivait pendant le court délai qui protège votre saisie et était ignorée — la fréquence restait donc sur l'ancienne bande jusqu'à ce qu'on touche le VFO.",