build(platformio): 3 recipes declare midi2 in lib_deps

Each PlatformIO recipe (ESP32_Host_MIDI-based, no Pico SDK) now
pulls midi2 from the PlatformIO Registry through lib_deps:

    lib_deps =
        sauloverissimo/midi2 @ ^0.3.3
        ...other deps...

Recipes touched:

- esp32-c6-devkitc-multi-midi2  (BLE + ESP-NOW, ESP32_Host_MIDI v6.0.1)
- esp32-s3-devkitc-host-midi2   (USB Host, ESP32_Host_MIDI v6.0.0)
- t-display-s3-shield-host-midi2 (USB Host + ST7789 piano, ESP32_Host_MIDI local)

These recipes already pulled midi2_cpp via lib_extra_dirs from the
parent tree; nothing about that path changes. midi2_cpp's own
src/midi2.{h,c} keeps the recipes building today and will go away in
the final cleanup commit, at which point lib_extra_dirs gives them
midi2_cpp source files only and lib_deps gives them midi2.

Author: sauloverissimo

examples/esp32-c6-devkitc-multi-midi2/pio/platformio.ini

@@ -53,6 +53,7 @@ build_flags =
 lib_extra_dirs = ../../..
 
 lib_deps =
+    sauloverissimo/midi2 @ ^0.3.3
     https://github.com/sauloverissimo/ESP32_Host_MIDI.git#v6.0.1
 
 ; Pre-build patch: ESP32_Host_MIDI v6.0.0 ships USB Host transport sources

examples/esp32-s3-devkitc-host-midi2/pio/platformio.ini

@@ -36,4 +36,5 @@ build_flags =
 lib_extra_dirs = ../../..
 
 lib_deps =
+    sauloverissimo/midi2 @ ^0.3.3
     https://github.com/sauloverissimo/ESP32_Host_MIDI.git#v6.0.0

examples/t-display-s3-shield-host-midi2/pio/platformio.ini

@@ -54,5 +54,6 @@ build_flags =
 lib_extra_dirs = ../../.., /home/shared/workspaces/github
 
 lib_deps =
+    sauloverissimo/midi2 @ ^0.3.3
     ESP32_Host_MIDI
     lovyan03/LovyanGFX@^1.2.0