feat(boards): add CH340 Uno clone source

[wingwogus]

Summary

• add a dedicated arduino.uno-r3-ch340 authoring source and sketch
• register the observed WCH CH340 USB identity 1a86:7523 plus common family variants
• keep detection confidence ambiguous because CH340 identifies a generic USB-serial bridge, not the board model

Firmware

• compile with the existing locked arduino:avr:uno toolchain
• produce the same deterministic firmware bytes as Arduino Uno R3
• intentionally omit generated HEX, manifest, and registry changes; the protected main publisher creates those after merge

Verification

• 26 firmware publisher tests pass
• generated-file PR policy passes
• deterministic double compilation passes (11211 bytes, SHA-256 b177f39bd3dcbbb817b853ba594796d69644e6ce407456f0ce0005b35bd149f1)