Fix bootloader hardware bring-up and add key-based boot entry

Hardware-tested the UF2 bootloader end to end on a real VersaPad v2
board. Found and fixed a real bug: the bootloader's jump into the app
(__set_MSP -> SCB->VTOR -> bx) hard-faulted on every standalone boot,
even with the debugger fully disconnected; identical register/VTOR
values injected directly by a halted debugger ran fine, which pointed
at the missing __DSB()/__ISB() barriers ARM's own guidance requires
for this exact pattern. Also fixed a USB PID collision (0x0011 is
Adafruit's own Gemma M0 bootloader PID, misidentified by Windows as a
Circuit Playground COM port instead of exposing VERSABOOT).

This board has no dedicated reset/boot button, so add a hardware boot
entry that doesn't need one: holding the bottom-right Cherry MX key
(key_id 24) during reset/power-on drives its matrix row and reads its
column directly in the bootloader, before the app is even validated.

Also corrected the app-side flash_with_bootloader.ld (was missing the
NVM carve-out flash_without_bootloader.ld already has) and
boards/versapad.json (wrong flash/RAM size, wrong MCU macro, stale
PID), and enabled the previously-commented-out env:versapad_usb.

Documented findings in bootloader/README.md, bootloader/TESTING.md,
and doc/09_known_limitations.md.

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

platformio.ini

@@ -23,7 +23,7 @@ extra_scripts     = upload_openocd.py
 debug_tool        = openocd
 
 ; ── USB SAM-BA (nur wenn Bootloader geflasht ist) ─────────────────────────────
-; [env:versapad_usb]
-; extends           = common
-; board             = versapad
-; upload_protocol   = sam-ba
+[env:versapad_usb]
+extends           = common
+board             = versapad
+upload_protocol   = sam-ba