Improve on-device logic (avoid C++ lambdas in YAML)

[jbunting]

C++ lambdas embedded in YAML are hard to maintain. Find a better approach for on-device logic (custom components, etc.).

Part of #1.

[jbunting]

Resolved. Established the pattern for externalizing on-device logic when it outgrows a clean one-liner: put it in a real C++ header (devices/common/firmware_status.h) and pull it in with esphome: includes: + a thin lambda binding — rather than a heavy custom component or a gnarly inline lambda. Piloted with the CalVer date math behind the "Firmware Days Behind" sensor (#8, PR #23).

The existing simple lambdas and vendored TubesZB logic were intentionally left as-is; this pattern is the go-to when future device logic gets complex.