docs: input integration guide (cockpit -> RIOJoy -> game)

Companion to OUTPUT-INTEGRATION.md: the three input surfaces (ViGEm x360
pad, SendInput scancode keyboard/mouse, RioGamepad HID), per-button routing
kinds incl. the fixed 11-button pad order, axis calibration + routing with
the triggers-are-buttons trap and the shipped Descent pattern, per-game
strategy, profile building/triggers workflow, benchless testing, checklist.
Cross-linked from README and the output guide.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -23,7 +23,8 @@ Red Planet — talk to the RIO directly and do not use this app.)
 | [`docs/PLAN.md`](docs/PLAN.md) | Full modernization plan |
 | [`docs/PROTOCOL.md`](docs/PROTOCOL.md) | RIO wire format + `iRIO` input-map reference |
 | [`docs/FEEDBACK.md`](docs/FEEDBACK.md) | Game→cockpit feedback endpoint (lamps + plasma over pipe/UDP, rumble) |
-| [`docs/OUTPUT-INTEGRATION.md`](docs/OUTPUT-INTEGRATION.md) | Integrator's guide: lamp address map, plasma display model, per-game recipes |
+| [`docs/INPUT-INTEGRATION.md`](docs/INPUT-INTEGRATION.md) | Integrator's guide: cockpit→game — routing kinds, pad/axis mapping, profile building |
+| [`docs/OUTPUT-INTEGRATION.md`](docs/OUTPUT-INTEGRATION.md) | Integrator's guide: game→cockpit — lamp address map, plasma display model, recipes |
 | _RIO board hardware & firmware_ | Moved to the [TeslaRel410 `restoration/`](https://gitea.mysticmachines.com/VWE/TeslaRel410/src/branch/main/restoration) archive — board photos, schematics, GAL decode (`restoration/rio-hardware`) and the RIO 4.3 board firmware (`restoration/rio-firmware`) |
 | [`docs/reference/`](docs/reference/) | Cockpit overlay art & the legacy labeling pipeline |
 | [`legacy/`](legacy/) | Original C++/vJoy implementation, kept as reference |