refactor(net): #13 Parsed.TrailerTruncated + diag logging

Code-quality review followup on Task 2 (becbde6) — addresses I1 (the
forward-looking concern that Tasks 3-9's inner-catch will leave partial
lists visible to callers with no signal) and M1 (silent inner catch).

Changes:
  - Parsed gains a trailing `bool TrailerTruncated` field. Both
    construction sites pass `false` by default; the trailer try/catch
    flips a local `trailerTruncated` to `true` on FormatException and
    feeds it into the final return.
  - Inner catch logs `pos`/`payload.Length`/exception message under
    ACDREAM_DUMP_VITALS=1, mirroring the outer catch's diagnostic
    pattern.
  - Task 2 test strengthened to assert defaults on Options2 /
    SpellbookFilters / HotbarSpells / DesiredComps / GameplayOptions /
    Equipped + TrailerTruncated=false (M2 followup — gives Tasks 3-9
    a regression guard if they consume into the wrong field).
  - New test `TryParse_TrailerAbsent_LessThan8BytesAfterEnchantments_*`
    documents the contract that <8 bytes after enchantments means the
    trailer is absent (not truncated): TrailerTruncated stays false,
    upstream attribute data survives.
  - Plan updated in lockstep so Tasks 3-11 implementers see the
    `trailerTruncated` local and the new return-arg position.

271/271 AcDream.Core.Net.Tests pass.

src/AcDream.Core.Net/Messages/PlayerDescriptionParser.cs

@@ -227,6 +227,13 @@ public static class PlayerDescriptionParser
         uint EquipLocation,
         uint Priority);
 
+    /// <summary>Result of <see cref="TryParse"/>. Trailer fields
+    /// (<c>OptionFlags</c> through <c>Equipped</c>) may be partially
+    /// populated when <see cref="TrailerTruncated"/> is <c>true</c> —
+    /// the parse degraded gracefully rather than discarding upstream
+    /// attribute / skill / spell / enchantment data. Callers that
+    /// require all-or-nothing trailer semantics should ignore the
+    /// trailer fields when this flag is set.</summary>
     public readonly record struct Parsed(
         uint WeenieType,
         DescriptionPropertyFlag PropertyFlags,
@@ -247,7 +254,8 @@ public static class PlayerDescriptionParser
         uint SpellbookFilters,
         ReadOnlyMemory<byte> GameplayOptions,
         IReadOnlyList<InventoryEntry> Inventory,
-        IReadOnlyList<EquippedEntry> Equipped);
+        IReadOnlyList<EquippedEntry> Equipped,
+        bool TrailerTruncated);
 
     /// <summary>
     /// Parse a PlayerDescription payload. The 0xF7B0 envelope has been
@@ -325,6 +333,7 @@ public static class PlayerDescriptionParser
             ReadOnlyMemory<byte> gameplayOptions = ReadOnlyMemory<byte>.Empty;
             List<InventoryEntry> inventory = new();
             List<EquippedEntry> equipped = new();
+            bool trailerTruncated = false;
 
             try
             {
@@ -334,9 +343,16 @@ public static class PlayerDescriptionParser
                     options1 = ReadU32(payload, ref pos);
                 }
             }
-            catch (FormatException)
+            catch (FormatException ex)
             {
-                // Trailer corrupted — keep what we have and return.
+                // Trailer corrupted — keep what we have and flag it. Once
+                // Tasks 3-9 add list reads inside this try block, partial
+                // lists may be visible to callers; TrailerTruncated tells
+                // them so they can ignore the trailer if they need all-or-
+                // nothing semantics.
+                trailerTruncated = true;
+                if (System.Environment.GetEnvironmentVariable("ACDREAM_DUMP_VITALS") == "1")
+                    System.Console.WriteLine($"PlayerDescriptionParser: trailer FormatException at pos={pos}/{payload.Length}: {ex.Message}");
             }
 
             return new Parsed(
@@ -344,7 +360,7 @@ public static class PlayerDescriptionParser
                 bundle, positions, attributes, skills, spells, enchantments,
                 optionFlags, options1, options2,
                 shortcuts, hotbarSpells, desiredComps, spellbookFilters,
-                gameplayOptions, inventory, equipped);
+                gameplayOptions, inventory, equipped, trailerTruncated);
         }
         catch (FormatException ex)
         {
@@ -373,7 +389,8 @@ public static class PlayerDescriptionParser
             0u,
             ReadOnlyMemory<byte>.Empty,
             System.Array.Empty<InventoryEntry>(),
-            System.Array.Empty<EquippedEntry>());
+            System.Array.Empty<EquippedEntry>(),
+            TrailerTruncated: false);
     }
 
     // ── Attribute block reader ──────────────────────────────────────────────