docs: list ProMicro solo build, document CARDKB_I2C, update release notes

- README's env table and external_keyboard.md's device-support table both
  get a ProMicro row -- missed when the PR #31 merge landed since neither
  was in scope for that commit.
- external_keyboard.md and build_flags.md now cover CARDKB_I2C alongside
  ENV_PIN_SDA/ENV_PIN_SCL: the PR generalised CardKB bus selection to a
  single macro after build_flags.md was first written, so the doc only had
  the older (still valid, but no longer the only) form.
- release-notes.md: added the ProMicro solo build and the new Build Flags
  reference page to v1.26's What's New.

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

docs/solo_features/build_flags.md

@@ -48,7 +48,8 @@ existing `solo/<board>/platformio.ini` for what's already claimed).
 
 | Flag | Adds |
 | --- | --- |
-| `ENV_PIN_SDA` / `ENV_PIN_SCL` | CardKB (M5Stack I2C keyboard, addr `0x5F`) on a second I2C bus. Probed at boot — harmless with nothing plugged in. See [External Keyboard & Joystick](./external_keyboard.md). |
+| `ENV_PIN_SDA` / `ENV_PIN_SCL` | CardKB (M5Stack I2C keyboard, addr `0x5F`) on a second I2C bus (resolves to `Wire1`). Probed at boot — harmless with nothing plugged in. See [External Keyboard & Joystick](./external_keyboard.md). |
+| `CARDKB_I2C=<Wire\|Wire1>` | Same CardKB support, naming the bus directly — for a board with no free pins for a second bus, set `CARDKB_I2C=Wire` to share the primary bus (already used by the display/RTC) instead of defining `ENV_PIN_SDA`/`ENV_PIN_SCL`. Takes precedence if both are somehow set. |
 | `UI_HAS_JOYSTICK=1` + `UI_HAS_JOYSTICK_UPDOWN=1` (optional) + `JOYSTICK_UP` / `JOYSTICK_DOWN` / `JOYSTICK_LEFT` / `JOYSTICK_RIGHT` + `PIN_USER_BTN` + `PIN_BACK_BTN` | A wired joystick (four direction contacts + a press contact for Enter). Replaces single-button navigation entirely once enabled. See [External Keyboard & Joystick](./external_keyboard.md). |
 | `PIN_GPIO1` .. `PIN_GPIO4` | Up to four general-purpose pins, each independently switchable between Off / Input / Output (GPIO1/GPIO2 also get an Analog step, if wired to an ADC-capable pin) from Tools › GPIO, and via the `!gpio1`..`!gpio4` bot commands. Not restricted to any particular board — works anywhere the pins are actually free. See [Tools Screen › GPIO](./tools_screen/tools_screen.md#gpio). |
 | `PIN_HALL_SENSOR` + `HALL_ACTIVE_HIGH=1` (optional) | A Hall-effect or reed sensor for a magnetic flip cover: closing locks and blanks the screen instantly, opening unlocks and wakes it, no combo either way. `HALL_ACTIVE_HIGH` is only for a module wired to pull the pin high (rather than low) when the magnet is near. See [Screen Lock › Magnetic cover](./screen_lock/screen_lock.md#magnetic-cover-hall-sensor). |

docs/solo_features/external_keyboard.md

@@ -23,6 +23,7 @@ see [Built-in keyboards](#built-in-keyboards-cardputer-adv-t-echo-lite--keyshiel
 | Heltec V4 *(experimental)* | ✅ solder to free GPIOs | ✅ solder to free GPIOs |
 | M5Stack Cardputer ADV *(experimental)* | — | built-in keyboard instead, see below |
 | LilyGO T-Echo Lite + KeyShield *(experimental)* | — | built-in keypad instead, see below |
+| ProMicro (nRF52840) *(experimental)* | ✅ shares the primary I2C bus (D8/D7) — no free pins for a second bus | — |
 
 ---
 
@@ -35,6 +36,11 @@ for it once at boot — nothing to enable in Settings.
 The same bus is scanned for environment sensors, so a CardKB and a sensor can
 share it.
 
+On a board with no free pins for a second bus (ProMicro), CardKB instead
+shares the primary bus already used by the display/RTC — set `CARDKB_I2C=Wire`
+as a build flag rather than `ENV_PIN_SDA`/`ENV_PIN_SCL`. See
+[Build Flags](./build_flags.md) for both forms.
+
 ### Typing
 
 Printable characters insert straight at the cursor, bypassing the on-screen grid