feat(nrf52): virtual filesystem mount points + XModem path routing

[benallfree]

This PR stacks on #10139 by introducing the beginnings of a virtual filesystem. I show how to use it via the XModem module, where I happen to need the feature. This allows callers to address storage by mount point rather than by direct FS reference.

Co-author: @GoatsAndMonkeys

Prerequisites

• Merge feat(nrf52): optional external QSPI LittleFS filesystem #10139
• Merge fix(xmodem): truncate file on open instead of appending #10143

Motivation

Meshtastic currently has one filesystem (FSCom) mapped to internal flash. With external flash now optionally mountable from #10139, it would help if there were a way to address it from a single API.

I'm thinking about these:

Prefix	Destination
/__int__/foo	Internal flash (InternalFS / LittleFS)
/__ext__/foo	External flash (extFS if mounted, else internal)
/__sd__/foo	SD card (reserved for future use, falls back to internal)
/foo	Internal flash — bare paths pass through unchanged

The double-underscore delimiters minimize collision risk with real file paths.

New public API:

FSRoute fsRoute(const char *path);     // resolve path → {FsMount, realPath}
File    fsOpenRead (const FSRoute &r);
File    fsOpenWrite(const FSRoute &r);
bool    fsRemove   (const FSRoute &r);
bool    fsMkdir    (const FSRoute &r);
bool    fsExists   (const FSRoute &r);

On platforms without external flash (ESP32, RP2040, STM32WL), all mount variants resolve to FSCom with the prefix stripped — identical behavior to today, zero breaking changes.

🤝 Attestations

• I have tested that my proposed changes behave as described.
• I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • Heltec (Lora32) V3
  • LilyGo T-Deck
  • LilyGo T-Beam
  • RAK WisBlock 4631
  • Seeed Studio T-1000E tracker card
  • Other (please specify below)