NRF52 LTO flags (#10655)

* Add LTO support for nrf52840 while preserving interrupt handlers

* nrf52840: enable whole-image LTO on all targets via nrf52_base

Moves -flto + the nrf52_lto.py exclusion middleware from the rak4631 env
(771018ca8) up to [nrf52_base], so every nrf52840 target inherits it.

nrf52_lto.py keeps the interrupt handlers out of LTO (framework core +
TinyUSB USBD_IRQHandler) -- they're referenced only from the asm vector
table, so whole-program LTO would otherwise drop them and the chip hangs.

HW-validated: RAK4631 (SX1262, -60KB) and muzi-base (LR1121) both boot and
init their radios. Build-verified on canaryone (fresh board, base-inherited).

Caveat: the RAK "1-Watt" variant's radio does not tolerate global-LTO
(SX126x init fails); it shares the rak4631 binary, so keep that hardware on
src-only or split it into a separate target.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* nrf52840: move -fmerge-all-constants to nrf52_base (all targets)

It had been trialed on rak4631 only; it's a general image-wide flag (the
same one stm32 uses globally, ~0.7KB), so move it up to [nrf52_base] next
to -flto so every nrf52840 target gets it instead of just rak4631.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* nrf52_lto.py: normalize path separators for Windows (Copilot review)

get_abspath() returns backslash-separated paths on Windows, so the
"/FrameworkArduino/" / "/cores/nRF5/" substring matches would miss and the
ISR-owning objects would still be LTO'd -> hang on first IRQ. Replace
backslashes with forward slashes before matching. No-op on macOS/Linux.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Fix  directory handling for LTO

* Add post-link guard to check for dropped ISR handlers in nrf52 LTO

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Jason P <applewiz@mac.com>

variants/nrf52840/nrf52.ini

@@ -14,6 +14,7 @@ platform_packages =
 extra_scripts =
   ${env.extra_scripts}
   extra_scripts/nrf52_extra.py
+  pre:extra_scripts/nrf52_lto.py
 
 build_type = release
 build_flags =
@@ -26,6 +27,8 @@ build_flags =
   -DMESHTASTIC_EXCLUDE_PAXCOUNTER=1
   -Os
   -std=gnu++17
+  -flto                 ; whole-image LTO (~-60KB) on every nrf52840 target; nrf52_lto.py (pre: extra_script) keeps the interrupt handlers out of LTO so they survive
+  -fmerge-all-constants ; fold identical constants image-wide (~0.7KB; same flag stm32 uses)
 build_unflags =
   -Ofast
   -Og