Build the library as a single translation unit

[ainyan03]

Why

Every implementation file pulls in the same set of framework headers, and the Arduino build pre-scans every source file for includes before compiling it, so that header parsing happened about 70 times per build. On Windows each pass costs around 3 s.

This PR builds the library as one translation unit: the .cpp files become .inl and are included from src/M5Unified.cpp.

Measured with arduino-cli, CoreS3 sketch, sequential build:

	Windows before	Windows after	macOS before	macOS after
Compiling library "M5Unified"	71 s	6 s	10 s	2 s

(The detection pass shrinks by the same 35 files.)

What changed

• src/**/*.cpp → *.inl, included from the new hub src/M5Unified.cpp (36 → 1 translation unit; the former M5Unified.cpp is now M5Unified.inl). The hub file carries the maintenance notes.
• File-local names that collided once they shared a scope:
  • led_count in LED_PaperMono / LED_PowerHub: private static constexpr member of each class
  • bcd2ToByte / byteToBcd2, duplicated in three RTC drivers: protected static helpers of RTC_Base
  • the I2S helpers and handle arrays of Speaker_Class / Mic_Class: _spk_i2s_* / _mic_i2s_* (the two arrays are distinct on purpose: TX and RX channels of the same port)
  • i2c_freq and the shared on-board device addresses: utility/m5unified_i2c_addr.hpp
  • IS_BIT_SET (AXP2101): #undef at the end of the file
• CMakeLists.txt lists the hub instead of globbing *.cpp.
• Each *.inl checks for M5UNIFIED_IMPLEMENTATION, which the hub defines around its includes, and stops with a clear #error when included on its own (a direct include used to surface only as duplicate symbols at link time).

Compatibility

• Arduino IDE, PlatformIO and ESP-IDF users need no changes. Custom build scripts that glob src/**/*.cpp will now pick up M5Unified.cpp only, which is the intended set.
• Public headers: two private constants and two protected static helpers were added; no class layout or virtual table changes, so downstream libraries that derive from these classes are unaffected.
• Binary size is unchanged within a few hundred bytes.

Verification

• Arduino builds for esp32 / esp32s2 / esp32s3 / esp32c3 / esp32c6 / esp32p4 (core 3.x) with the same warnings as develop; ESP-IDF 5.5 build; native SDL build.
• The CI of this repository run on a fork of this branch: Arduino Build and ESP-IDF Build pass.