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.

docs/superpowers/plans/2026-05-10-issue-13-pd-trailer.md

@@ -154,9 +154,17 @@ public readonly record struct Parsed(
     uint SpellbookFilters,
     ReadOnlyMemory<byte> GameplayOptions,
     IReadOnlyList<InventoryEntry> Inventory,
-    IReadOnlyList<EquippedEntry> Equipped);
+    IReadOnlyList<EquippedEntry> Equipped,
+    bool TrailerTruncated);
 ```
 
+> **Code-review followup (added after Task 2 review):** the trailing
+> `TrailerTruncated` flag was added to let callers distinguish a clean
+> parse from one where the trailer try/catch swallowed a `FormatException`
+> mid-section (Tasks 3–9 will make this reachable). All construction sites
+> pass `TrailerTruncated: false` by default; the trailer try/catch in
+> `TryParse` flips a local to `true` on catch.
+
 - [ ] **Step 3: Update `BuildPartial` to fill the new fields with defaults.**
 
 Replace the body of `BuildPartial` (~line 275) with:
@@ -179,7 +187,8 @@ private static Parsed BuildPartial(
         0u,
         ReadOnlyMemory<byte>.Empty,
         System.Array.Empty<InventoryEntry>(),
-        System.Array.Empty<EquippedEntry>());
+        System.Array.Empty<EquippedEntry>(),
+        TrailerTruncated: false);
 }
 ```
 
@@ -198,7 +207,8 @@ return new Parsed(
     0u,
     ReadOnlyMemory<byte>.Empty,
     System.Array.Empty<InventoryEntry>(),
-    System.Array.Empty<EquippedEntry>());
+    System.Array.Empty<EquippedEntry>(),
+    TrailerTruncated: false);
 ```
 
 - [ ] **Step 5: Run the build + existing tests to verify no regressions.**
@@ -283,6 +293,7 @@ List<(uint, uint)> desiredComps = new();
 ReadOnlyMemory<byte> gameplayOptions = ReadOnlyMemory<byte>.Empty;
 List<InventoryEntry> inventory = new();
 List<EquippedEntry> equipped = new();
+bool trailerTruncated = false;
 
 try
 {
@@ -292,9 +303,14 @@ try
         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. Tasks 3-9
+    // can leave partial lists in scope; TrailerTruncated lets callers
+    // ignore the trailer when 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(
@@ -302,9 +318,14 @@ return new Parsed(
     bundle, positions, attributes, skills, spells, enchantments,
     optionFlags, options1, options2,
     shortcuts, hotbarSpells, desiredComps, spellbookFilters,
-    gameplayOptions, inventory, equipped);
+    gameplayOptions, inventory, equipped, trailerTruncated);
 ```
 
+> **Tasks 3–9 note:** every `return new Parsed(...)` extension or
+> rewrite in subsequent tasks must include `trailerTruncated` as the
+> final positional argument, and any new try-blocks that read trailer
+> sections should set `trailerTruncated = true;` in their catch.
+
 - [ ] **Step 4: Run the test — expect PASS.**
 
 Run: `dotnet test --filter "FullyQualifiedName~TryParse_TrailerOptionFlagsAndOptions1"`