release: v1.19.4 — extract the SMC plumbing into a shared SMCClient

An internal release: the SMC plumbing moved out of the fan reader into a shared client. Nothing you
can see changed, and that is the whole acceptance criterion.

Fixed

• The SMC connection is now shared, so a second sensor source can't open a second one. Reading
  fan RPM opens a connection to the System Management Controller, and that connection is never
  closed — the app holds it for its whole life and the kernel reclaims it at exit. That is fine for
  one. But the plumbing was private to the fan reader, so the next SMC-backed source (die
  temperature) would have had to duplicate it and open its own, leaving two permanently-open
  connections where one would do. The mechanism now lives in a single shared SMCClient, and the
  struct-layout safety check that disables the SMC path on an incompatible toolchain now gates
  every SMC source at once instead of only the reader holding its own copy of the check.

Unchanged, deliberately

• Fan readings, the per-fan menu lines, availability on fanless Macs, and every CLI flag and
  environment variable. The fan label and menu rows were captured from the pre- and post-change
  builds back-to-back and are identical, at two hardware states (fans warm, and both fans stopped).