exercises: add Exercise 07 SD startup watcher with OLED status and hot-insert/removal handling

exercises/07_SD_Startup_Watcher/README.md

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+## Exercise 07: SD Startup Watcher
+
+This exercise is derived from `Exercise 05` and keeps that original exercise intact.
+The focus here is isolating reusable SD startup and hot-insert watcher logic into a library-style structure.
+
+This exercise now has two parts:
+
+1. A reusable SD startup/watcher library in `lib/startup_sd`.
+2. A harness app in `src/main.cpp` that demonstrates how to use that library.
+
+Watcher behavior:
+
+1. Initializes PMU and enables SD power rail (AXP2101 BLDO1).
+2. Polls for card changes with debounced state transitions.
+3. Emits events only on change:
+   - `EVENT: card inserted/mounted`
+   - `EVENT: card removed/unavailable`
+   - `EVENT: no card detected`
+4. On mount event, emits callback status (`SdEvent`) and runs SD write workflow.
+5. Every 15 seconds while mounted, runs a periodic write/permission check.
+6. Uses fast preferred probe (`HSPI @ 400k`) and occasional full fallback scan.
+
+Status callback usage:
+
+- `SdEvent::NO_CARD` -> show "Missing SD card / Please insert card to proceed"
+- `SdEvent::CARD_MOUNTED` -> card ready
+- `SdEvent::CARD_REMOVED` -> card removed, wait for insert
+
+Files used in this exercise:
+- `/Exercise_07_test.txt`
+- `/test/testsub1/testsubsub1/Exercise_07_test.txt`
+
+## Build
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a
+```
+
+## Upload
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a -t upload --upload-port /dev/ttyACM0
+```
+
+## Monitor
+
+```bash
+screen /dev/ttyACM0 115200
+```