Unify base-path and relative-path handling across all filesystem adap… - #14

…ters

Every adapter now resolves paths the same way: a path is always relative to the adapter's root ("/a", "./a" and "a" are the same file) and the base path says where that root is. An empty base selects the backend's default root: the current directory on native hosts, the fallback mount on ESP32, the filesystem root elsewhere.

• Add allocation-free Path.h (normalize/join/copy_base/persistent_mount)
• Honor the base path in the POSIX and stdio adapters
• Accept relative paths on the ESP32 LittleFS/SPIFFS/SD adapters, which also stops init()'s write check from reformatting on every boot
• Copy base paths into fixed adapter buffers and keep the LittleFS mount point in static storage, since Arduino's FS layer keeps the pointer
• Native behavior change: with the default base, absolute paths now resolve under the current directory; pass "/" for host-absolute paths

Problem

Solution

Compatibility

• No change to the on-disk format (or: a compatibility/migration path is described below and added to CHANGELOG)
• Crash safety unchanged (or: power-loss behavior at each step is explained below)
• No breaking public API change (or: described below and added to CHANGELOG)
• Still header-only, C++14, no new dependencies
• No board- or variant-specific code; any platform-specific code is unavoidable, isolated (adapter or guard), and explained below

Testing

Platforms tested (and how):

• native (pio test -e native)
• ESP32 (board / filesystem: )
• nRF52 (board / filesystem: )

Platforms not tested, and why:

Docs

• README / docs/ / CHANGELOG updated where user-visible behavior, build flags, configuration macros or APIs changed