feat: add typed BH1750 read tool

[tnm]

Summary

• add a first-class bh1750_read built-in tool for typed ambient light reads over I2C
• extend host I2C mocks/tests to cover BH1750 success and failure paths, including distinct measurement-start failure coverage
• update user-facing docs and the provider harness so BH1750 shows up wherever supported tools are listed

Details

• bh1750_read accepts sda_pin, scl_pin, optional address, and optional frequency_hz
• it defaults to address 0x23 and also supports 0x5C
• runtime flow is deterministic in firmware: power on, start one-time high-resolution measurement, wait 180 ms, read 2 bytes, format lux
• raw i2c_* tools stay in place; this is the first typed read layer on top

Validation

• ./scripts/test.sh host
• ./scripts/build.sh
• flashed feat/bh1750-read onto the attached board and confirmed post-flash serial status smoke still works

Caveat

• I do not have a convincing positive hardware BH1750 read on the attached board. This branch has strong host coverage plus generic board smoke, but not a real BH1750-on-hardware proof on the currently attached setup.