Sync v1.6.2: perf, audit fixes, status cleanup

[duczz]

Summary

Syncs the public repository with the latest private-repo integration state (v1.6.2).

Code Changes

• perf (P-1): Decouple calendar/schedule fetch to avoid blocking the coordinator loop
• perf (P-2): Eliminate double update when switches toggle (force_comfort / force_eco / party / vacation)
• fix: Heartbeat now rearms on a 60s interval (was one-shot)
• fix: Reboot-detection logs deduplicated (once per entity per boot)
• fix: Status label — Away (🚶 -X°C) renamed to Comfort (🚶 -X°C) when the comfort schedule is active (the base is still comfort temperature, just reduced)
• fix: Sunshine offset shown in Away status when both are active
• refactor (B-4 / B-8): Remove orphaned calendar fields, extract _score_event() helper
• diagnostics: last_update now filled with useful structured data (timestamp, mode, target, reason, changes)

Docs

• Admin handbook: documents FRITZ!DECT radio latency (1–3s) after switch toggles
• README: feature list refreshed for v1.6.2

Out of scope (public-only)

• .github/workflows/validate.yml (has its own hassfest job) — unchanged
• LICENSE — unchanged
• .gitattributes, .gitignore, hacs.json, .github/* templates — identical content, unchanged