docs: record once-and-for-all decisions (single host, cached IP, robust WoL)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

DECISIONS.md

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+# LG TV Bridge — Decisions
+
+Format: `YYYY-MM-DD: <decision> — <why>`
+
+- **2026-06-01: Single bridge, single host — runs on steel141, pve173 retired.**
+  The bridge had been running on pve173 (always-on node) since 2026-03-23,
+  SSHing to steel141 for every TV op. Consolidated everything onto steel141.
+  *Why:* the WebOS pairing key (`/home/seth/.aiopylgtv.sqlite`) lives only on
+  steel141, so `power_off` always routes through it regardless of bridge host —
+  pve173 added an SSH hop and a second host for no real availability gain (the
+  TV is uncontrollable whenever steel141 is down anyway). One host, no SSH hop.
+
+- **2026-06-01: Cached-IP discovery, sweep only on miss.** Persist last-known TV
+  IP in `~/.cache/lgtv/last_ip`; probe it directly (TCP :3000/:3001). Full /24
+  ping-sweep runs only on a cache miss, then re-caches. *Why:* the old code
+  ping-swept all 254 hosts over SSH on every 30s HA state poll — slow, network
+  spam, and racy (stale ARP) → the chronic "flaky state in HA" symptom.
+
+- **2026-06-01: Robust WoL — unicast + broadcast, ports 9/7, 3 bursts.** *Why:*
+  old code sent broadcast-only on ports 9/1235 once. Unreliable wakes. NOTE: the
+  remaining wake failures are TV-side (Quick Start+ / Mobile TV On must be ON;
+  wired Ethernet far more reliable than Wi-Fi for LG WoL) — not fixable in code.
+
+- **2026-06-01: Deps via `pip --user --break-system-packages` for `seth`.** paho
+  + bscpylgtv live in `/home/seth/.local` (PEP 668 box). Matches how bscpylgtv
+  was already installed; the systemd unit runs `User=seth`.
+
+## Deferred / Rejected
+- **DHCP reservation for the TV (static IP):** not done — Pi-hole doesn't run
+  DHCP (router does), so it's Seth's action on the router. The cached-IP probe
+  makes it unnecessary for correctness; a reservation would just let us drop the
+  sweep-fallback entirely. Add if convenient.
+- **Unique per-instance MQTT client_id:** rejected — keeping a fixed
+  `lgtv-bridge` id means a duplicate instance is *loud* (reconnect war in broker
+  logs) rather than silent. Single-instance discipline is the real guard. (A
+  duplicate on 2026-06-01 during this very migration is what surfaced all this.)