docs(api): rewrite KOReader bookmark sync for current protocol

sync_bookmarks.md documented a request shape the handler never accepted.

- Document the real body: book_uuid/book_sha256 (either required,
  SHA-256 is format-aware), plus separate bookmarks/notes/highlights
  arrays using the shared KOReader annotation shape (pos0/pos1, page,
  text, type, per-annotation book_sha256, dedup_key, percentage)
- Document color semantics from 178fb2e/dafcadd: KOReader palette
  names map to web hex swatches at the boundary, echoes carry no color
  so stored web colors survive round-trips, explicit colors are device
  edits
- koreader-protocol.md: cross-link the bookmark shape/color/dedup
  rules from the progress-sync field table

docs/developer/api/sync/koreader-protocol.md

@@ -32,7 +32,7 @@ KOReader uses a custom JSON-based sync protocol.
 | books[].chapter    | integer | No       | Current chapter                                      |
 | books[].epubcfi    | string  | No       | EPUB CFI location                                    |
 | books[].character  | integer | No       | Character offset                                     |
-| books[].bookmarks  | array   | No       | Array of bookmarks/highlights                        |
+| books[].bookmarks  | array   | No       | Array of bookmarks/highlights (shape, color mapping, and echo/dedup rules: see [Sync Bookmarks](../koreader/sync_bookmarks.md)) |
 
 \* At least one of `uuid` or `sha256` should be present. The server resolves the
 book through the shared `BookResolver` with this priority: `uuid` → `sha256` →