fix: get_thumbnail size limits, default=small, quality quirk docs (v0.3.5)

- Default size changed large → small (avoids MCP buffer overflows)
- Hard limit: return Error: when thumbnail exceeds ~2 MB base64 (1.5 MB raw)
- Soft limit: add "warning" field to JSON when thumbnail exceeds ~500 KB base64
  (375 KB raw), advising to use size='small'
- Constants _THUMB_ABORT_BYTES / _THUMB_WARN_BYTES moved to module level
- 6 new tests for size cap/warning/default/DSM-error paths (113 total)
- SPEC.md: document quality-ignored quirk, size ranges, soft+hard limits
- CLAUDE.md: DSM Quirks entry for Thumb quality/size behaviour

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -186,6 +186,15 @@ See [SPEC.md](SPEC.md) for full tool specifications and DSM call details.
   maps error 400 to a clear "requires Btrfs-formatted volume" message.
 - **`SYNO.FileStation.BackgroundTask`:** Only the `list` method is available (v1–v3).
   No stop/cancel/clear methods exist on this firmware.
+- **`SYNO.FileStation.Thumb` — `quality` parameter ignored:** DSM accepts the `quality`
+  field in the POST body but always returns the same JPEG regardless of the value.
+  No server-side quality control is available.
+- **`SYNO.FileStation.Thumb` — size limits:** `small` thumbnails range from ~5 KB to
+  ~548 KB raw depending on source image resolution. `medium`/`large` can exceed 380 KB
+  raw. `original` reflects the full stored image and may be several MB.
+  The `get_thumbnail` tool enforces: abort >1.5 MB raw (≈2 MB base64), warning >375 KB
+  raw (≈500 KB base64). Default changed from `large` to `small` to avoid MCP buffer
+  overflows.
 
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