feat(face-diagnostic): Phase 4 render, command, and init wiring

Phase 4 completes the face/font diagnostic. cj/describe-face-at-point renders cj/--face-diagnosis-at into a read-only *Face Diagnosis* buffer (cj/face-diagnostic-mode), with a region-scan mode over distinct face-runs (capped at 20) and an out-of-scope banner. It is required in init.el. The render is split into small section formatters tested on captured plists, and the command is smoke-tested and live-verified in the daemon, where it already surfaces the active auto-dim remaps. The command name is settled as cj/describe-face-at-point. The keybinding stays Craig's pick, and face-name buttons plus the module-header allowlist entry are filed as a follow-up. The spec is marked implemented and renamed to its lifecycle filename. 35 ERT tests, byte-compile clean.

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