perf(watch-history): ✨ stop the background poll freezing the main thread

The 8s watch-history poll ran refresh() on the main actor, which read and
JSON-decoded the unioned watch log THREE times (playedPaths, resumePositions,
episodeProgress each re-read) and called MediaPaths.toRemote() per event — and
every toRemote rebuilt ProcessInfo.environment (~22µs each, the whole env dict
is reconstructed on every access) plus a homeDirectory lookup. A live sample
caught the main thread 100% in this path; the app sat at 78–113% CPU.

- Cache MediaPaths.remoteRoot / mappings (process-constant) → kills the
  per-call env-dictionary rebuild storm.
- WatchHistory.derivedState(): read+decode the log ONCE, feed all three
  derived computations → 3× fewer reads/decodes per refresh.
- WatchHistoryController.refreshAsync(): the background poll now parses off the
  main thread on a utility task and only assigns the small results on main.

Settled CPU drops from ~78% sustained to ~0% idle.

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

Sources/TVAnarchyCore/Library/WatchHistory.swift

@@ -48,12 +48,29 @@ public enum WatchHistory {
         return f
     }
 
+    /// Read+decode the unioned watch logs **once** and compute every derived set the
+    /// controller needs. Returns only public result types (so it crosses the type
+    /// boundary to `WatchHistoryController`); the `[WatchEvent]` array stays private
+    /// inside this enum. Replaces three separate read+decode passes per refresh.
+    static func derivedState() -> (played: Set<String>,
+                                   resume: [String: Double],
+                                   episodes: [String: EpisodeProgress]) {
+        let events = readWatchlog()
+        return (playedPaths(from: events),
+                resumePositions(from: events),
+                episodeProgress(from: events))
+    }
+
     public static func continueItems(limit: Int = 24) -> [ContinueItem] {
+        continueItems(from: readWatchlog(), limit: limit)
+    }
+
+    private static func continueItems(from events: [WatchEvent], limit: Int = 24) -> [ContinueItem] {
         var byKey: [String: ContinueItem] = [:]
         let iso = ISO8601DateFormatter()
         let isoFrac = fractionalFormatter()
 
-        for ev in readWatchlog() {
+        for ev in events {
             let when = parseTS(ev.ts, iso, isoFrac)
             let item = ContinueItem(
                 title: ev.label.isEmpty ? ev.show : ev.label,
@@ -75,8 +92,12 @@ public enum WatchHistory {
     }
 
     public static func resumePositions() -> [String: Double] {
+        resumePositions(from: readWatchlog())
+    }
+
+    private static func resumePositions(from events: [WatchEvent]) -> [String: Double] {
         var out: [String: Double] = [:]
-        for item in continueItems(limit: 2000) {
+        for item in continueItems(from: events, limit: 2000) {
             if let p = item.positionSeconds, p > 1 { out[MediaPaths.toRemote(item.path)] = p }
         }
         return out
@@ -99,10 +120,14 @@ public enum WatchHistory {
     }
 
     public static func episodeProgress() -> [String: EpisodeProgress] {
+        episodeProgress(from: readWatchlog())
+    }
+
+    private static func episodeProgress(from events: [WatchEvent]) -> [String: EpisodeProgress] {
         var best: [String: WatchEvent] = [:]
         let iso = ISO8601DateFormatter()
         let isoFrac = fractionalFormatter()
-        for ev in readWatchlog() where isRealVideo(ev.path) {
+        for ev in events where isRealVideo(ev.path) {
             guard let pos = ev.resumeSeconds, pos >= 0 else { continue }
             let key = MediaPaths.toRemote(ev.path)
             if let prev = best[key] {
@@ -122,8 +147,11 @@ public enum WatchHistory {
     }
 
     public static func playedPaths() -> Set<String> {
+        playedPaths(from: readWatchlog())
+    }
+
+    private static func playedPaths(from events: [WatchEvent]) -> Set<String> {
         var out = Set<String>()
-        let events = readWatchlog()
         // Compute last reset per show so a rewatch clears the "started" set for
         // badges and nextUnwatched while preserving append-only history.
         var lastResetByShow: [String: Date] = [:]
@@ -332,11 +360,28 @@ public final class WatchHistoryController {
     }
 
     /// Re-parse the unioned watch logs (plum + black mirror) and recompute all derived sets.
-    /// Cheap; called after our own appends and on the background poll to catch externals.
+    /// Reads+decodes the log **once** and feeds it to every derived computation (the old
+    /// path read and JSON-decoded the whole log three times). Synchronous — used right
+    /// after our own appends so callers see fresh state immediately; the background poll
+    /// uses `refreshAsync()` instead to keep this work off the main thread.
     public func refresh() {
-        playedPaths = WatchHistory.playedPaths()
-        resumePositions = WatchHistory.resumePositions()
-        episodeProgress = WatchHistory.episodeProgress()
+        let d = WatchHistory.derivedState()
+        playedPaths = d.played
+        resumePositions = d.resume
+        episodeProgress = d.episodes
+        lastRefresh = Date()
+    }
+
+    /// Off-main refresh for the background poll: the read+decode+derive runs on a
+    /// utility thread, and only the (small) result assignment happens on the main
+    /// actor. This is what stops the 8-second poll from periodically freezing the UI.
+    public func refreshAsync() async {
+        let derived = await Task.detached(priority: .utility) {
+            WatchHistory.derivedState()
+        }.value
+        playedPaths = derived.played
+        resumePositions = derived.resume
+        episodeProgress = derived.episodes
         lastRefresh = Date()
     }
 
@@ -390,7 +435,7 @@ public final class WatchHistoryController {
         pollTask?.cancel()
         pollTask = Task { [weak self] in
             while !Task.isCancelled {
-                self?.refresh()
+                await self?.refreshAsync()
                 // Occasional black sync (the call itself throttles internally)
                 _ = await self?.syncBlack()
                 try? await Task.sleep(for: .seconds(8))

Sources/TVAnarchyCore/MediaPaths.swift

@@ -12,21 +12,25 @@ import Foundation
 /// (it's the identity on paths that are already storage-side).
 public enum MediaPaths {
     /// Storage-side media root (env-overridable, matches the TS bridge default).
-    public static var remoteRoot: String {
+    /// Cached once: the environment is fixed for the process lifetime, and
+    /// `ProcessInfo.environment` rebuilds the entire env dictionary on every access
+    /// (~22µs), which `toRemote` would otherwise pay thousands of times per watch-log
+    /// refresh. See the watch-history poll hot path.
+    public static let remoteRoot: String =
         ProcessInfo.processInfo.environment["BLACK_MEDIA_ROOT"] ?? "/bigdisk/_/media"
-    }
 
     /// Legacy laptop mount prefix → storage-side absolute prefix. Longest first so
     /// `~/_/bigdisk/_/media` wins over `~/_/bigdisk`. Only relevant for stale paths
     /// persisted before the mount was dropped — fresh scans are already storage-side.
-    private static var mappings: [(plum: String, remote: String)] {
+    /// Cached once (the home dir and root are process-constant) for the same reason.
+    private static let mappings: [(plum: String, remote: String)] = {
         let home = FileManager.default.homeDirectoryForCurrentUser.path
         return [
             (home + "/_/bigdisk/_/media", remoteRoot),
             (home + "/media", remoteRoot),
             (home + "/_/bigdisk", "/bigdisk"),
         ]
-    }
+    }()
 
     /// Normalize any path to its storage-side absolute form. Already-canonical paths and
     /// anything we don't manage pass through unchanged (so it's the identity on the