Centralize portable settings persistence

[BanjoR]

Summary

This PR defines a platform-neutral persistence boundary for the existing settings store. It is scoped to the persistence access layer and preserves existing settings compatibility.

Related issues:

• DCC-EX/CommandStation-EX#367
• DCC-EX/CommandStation-EX#485

Superseded fork-only PR: BanjoR/CommandStation-EX#4, now closed in favor of this upstream-targeted PR.

Bounded scope

• Route existing settings reads, writes, and capacity queries through EEStore::read, EEStore::write, and EEStore::capacity.
• Permit a board-specific backend to provide EESTORE_READ, EESTORE_WRITE, and EESTORE_CAPACITY hooks while retaining Arduino EEPROM behavior by default.
• Preserve the legacy EEStoreData byte layout: DCC++1 identifier plus the three 16-bit object counts.
• Exclude settings migration, a new settings protocol, EEPROM record-stream redesign, automatic data erasure, WiFi credential policy, EXRAIL storage, and any unrelated protocol/traction/integration code.

Validation

Pass:

• tests/host_eestore_format.ps1 — passed: EEStore host format test passed.

• python -m platformio run -e mega2560 using isolated task-local PlatformIO directories — passed; firmware linked and produced HEX, RAM 1357/8192 (16.6%), Flash 58190/253952 (22.9%).

• git diff --check upstream/master...HEAD — passed.

• Final checkout cleanliness/path audit — passed: clean worktree; only the ten bounded files in the diff; no generated .pio, work, or config.h files; no forbidden/unrelated paths.

• Direct EEPROM access audit — passed: production call sites use the EEStore boundary; the only remaining EEPROM.* references are the documented default hook definitions.

• PASS - Exact default python -m platformio run in a clean task checkout with an isolated PlatformIO core completed for all five configured environments: mega2560, ESP32, Nucleo-F411RE, Nucleo-F446RE, and Nucleo-F429ZI.

Exact-head fork CI: PASS - CI run 31951891358 completed successfully for head 0a50786. The upstream PR has no firmware check attached because the main workflow is push-triggered. The C++ host test source contains compile-time layout assertions, but a native host C++ compiler is not installed in this environment; the AVR PlatformIO build validates the shared production format header instead.

Hardware validation

Not run—no hardware available.

Maintainer bench criteria: build with the intended board configuration; flash a Mega2560 or equivalent EEPROM-backed target; load at least one turnout, sensor, and output; issue <E>; power-cycle; verify the same counts and definitions reload; issue <e> and verify a subsequent reboot loads an empty store. For a non-EEPROM target, define the board backend hooks and repeat the same round-trip and clear tests, including capacity reporting and preservation of an existing legacy DCC++1 header.