feat(tabs): add IsPinned with separate pool and 5-tab cap

Tester-Request from Jin (2026-05-03): TempTabs should be pinnable so a
key conversation partner survives a relog. Right-click a TempTab and
choose Pin Tab / Unpin Tab / Promote to permanent.

Pool semantics:
- AutoTellTabsLimit (15) still gates the auto-managed unpinned pool.
- Pinned TempTabs live in their own pool, hard-capped at 5.
- The 6th pin attempt fails with a notification; users can unpin first
  or promote to permanent.
- Unpinning into a full unpinned pool drops the oldest unpinned (no
  user friction).

Mechanics:
- Tab.IsPinned (default false); Tab.Clone() carries it.
- Migration v16 -> v17 (additive; existing tabs default to unpinned).
- Three strip-sites synchronised through TabLifecycleHelpers:
  Plugin.cs load-time, Plugin.SaveConfig, Configuration.UpdateFrom.
- AutoTellTabsService:
  * MaxPinnedTempTabs constant.
  * F2.1 _activeTempTabCount counter retired — ActiveTempTabCount is
    now Tabs.Count(predicate). Pin/Unpin/Promote transitions are
    cold-path and don't need lock-free reads.
  * DropOldestTempTab filters on IsInUnpinnedPool so pinned tabs are
    never drop candidates.
  * OnLogout strips only the unpinned pool; pinned popouts and the
    active-tab switch behave correspondingly.
  * TryPin / Unpin / PromoteToPermanent service methods.
- ChatLogWindow tab context menu: Pin / Unpin / Promote with disabled-
  state at-cap tooltip + Promote tooltip explaining the channel-filter
  side effect.
- HellionStrings (EN+DE) for menu labels, tooltips, the limit warning.
- AutoTellTabsLimit slider description now flags the separate pinned
  pool so users aren't surprised by 18 tabs when the limit reads 15.

HellionChat/Plugin.cs

@@ -169,20 +169,22 @@ public sealed class Plugin : IAsyncDalamudPlugin
         PlatformUtil = new DalamudPlatformUtil();
         LogProxy = new DalamudPluginLogProxy(Log);
 
-        // Schema gate: v1.4.x requires config v16. Users on older schemas
-        // must install v1.4.2 first to run the migration chain.
+        // Schema gate: v1.4.x requires config v16+. Users on older schemas
+        // must install v1.4.2 first to run the migration chain. v17 adds
+        // Tab.IsPinned (additive, no data migration needed) so v16 configs
+        // load cleanly and get their Version stamp bumped after the gate.
         if (Config.Version < 16)
         {
             throw new InvalidOperationException(
-                $"HellionChat v1.4.6 requires config schema v16, got v{Config.Version}. "
-                    + "Please install v1.4.2 first to migrate the configuration, then upgrade to v1.4.6."
+                $"HellionChat v1.4.7 requires config schema v16, got v{Config.Version}. "
+                    + "Please install v1.4.2 first to migrate the configuration, then upgrade to v1.4.7."
             );
         }
+        Config.Version = 17;
 
-        // Session-only tabs are stripped on every load; AutoTellTabsService.Initialize
-        // then re-pegs TempTabCounter from the stripped list, not the pre-strip snapshot.
-        // TEST-MIRROR: ../../Hellion Build test/_Helpers/TempTabCounterTests.cs
-        Config.Tabs.RemoveAll(t => t.IsTempTab);
+        // Unpinned TempTabs are session-only and dropped on every load. Pinned
+        // TempTabs survive reload — Jin's tester feedback (v1.4.7).
+        Config.Tabs.RemoveAll(TabLifecycleHelpers.ShouldStripOnLoad);
 
         LanguageChanged(Interface.UiLanguage);
         ImGuiUtil.Initialize(this);
@@ -652,21 +654,17 @@ public sealed class Plugin : IAsyncDalamudPlugin
 
     internal void SaveConfig()
     {
-        // Session-only Auto-Tell-Tabs aren't persisted, so they move aside
-        // before serialization and re-attach after. Cloning only the temp
-        // subset keeps the allocation proportional to AutoTellTabsLimit
-        // (<=15) instead of the full tab list.
-        var tempTabs = Config.Tabs.Where(t => t.IsTempTab).ToList();
-        Config.Tabs.RemoveAll(t => t.IsTempTab);
+        // Only unpinned TempTabs are session-only — they move aside before
+        // serialization and re-attach after. Pinned TempTabs stay in
+        // Config.Tabs across the save so JSON includes them. Cloning only the
+        // unpinned subset keeps the allocation proportional to
+        // AutoTellTabsLimit (<=15) instead of the full tab list.
+        var unpinnedTempTabs = Config.Tabs.Where(TabLifecycleHelpers.IsInUnpinnedPool).ToList();
+        Config.Tabs.RemoveAll(TabLifecycleHelpers.ShouldStripOnSave);
 
         Interface.SavePluginConfig(Config);
 
-        Config.Tabs.AddRange(tempTabs);
-
-        // F2.1: the mid-step RemoveAll bypasses AutoTellTabsService, so
-        // re-peg the counter. Null-conditional because SaveConfig can fire
-        // before Phase-2 init.
-        AutoTellTabsService?.ResyncTempTabCounter();
+        Config.Tabs.AddRange(unpinnedTempTabs);
     }
 
     internal void LanguageChanged(string langCode)