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>

variants/versapad/linker_scripts/gcc/flash_with_bootloader.ld

@@ -6,7 +6,8 @@ SEARCH_DIR(.)
 
 MEMORY
 {
-  rom (rx)  : ORIGIN = 0x00002000, LENGTH = 0x0001E000  /* 120K (128K - 8K bootloader) */
+  rom (rx)  : ORIGIN = 0x00002000, LENGTH = 0x0001D900  /* 118.25K – Firmware (128K - 8K bootloader - 1.25K NVM) */
+  nvm (rx)  : ORIGIN = 0x0001FB00, LENGTH = 0x00000500  /* 1.25K   – Makros + Config, same layout as flash_without_bootloader.ld */
   ram (rwx) : ORIGIN = 0x20000000, LENGTH = 0x00004000  /* 16K  */
 }