build: add PlatformIO support

[zevorn]

Summary

Add PlatformIO as an alternative build system for rt-claw, enabling one-click build/flash/debug from VS Code and CLI without manual toolchain setup.

Motivation

• PlatformIO is widely used in the embedded community, lowering the barrier for new contributors
• Auto-manages toolchains, SDKs, and board definitions — no manual ESP-IDF installation needed
• Native VS Code integration (IntelliSense, one-click flash, serial monitor, debugging)
• Supports ESP32-C3 and ESP32-S3 targets via espressif32 platform

Scope

• Add platformio.ini with environments for ESP32-C3 and ESP32-S3 boards
• Integrate Meson-built static libraries (libclaw.a, libosal.a) into PlatformIO build flow
• Verify QEMU and real hardware flash/debug workflows
• Add documentation in docs/en/ and docs/zh/

Notes

PlatformIO coexists with the current Meson + CMake build — no changes to existing build paths.