docs: fix RichtextState marks encoding and ContainerArena i32 format

1. EncodedMark is postcard Vec (row-wise), not columnar
   - The marks field in EncodedText lacks #[columnar(class = "vec")]
   - Only spans has columnar encoding via that attribute

2. ContainerArena key_idx_or_counter uses zigzag varint, not SLEB128
   - Row-wise postcard encoding uses zigzag for signed integers

Author: claude

docs/encoding-container-states.md

@@ -230,7 +230,7 @@ Richtext container stores text with optional styling marks.
 ├─────────────────┼──────────────────────────────────────────────────────────┤
 │ Vec<String>     │ Style keys (postcard Vec<InternalString>)                │
 ├─────────────────┼──────────────────────────────────────────────────────────┤
-│ Vec<Mark>       │ EncodedMark array (columnar)                             │
+│ Vec<Mark>       │ EncodedMark array (postcard Vec, row-wise)               │
 └─────────────────┴──────────────────────────────────────────────────────────┘
 ```
 
@@ -252,18 +252,18 @@ len interpretation:
   - -1: Style mark end
 ```
 
-### EncodedMark (Columnar)
+### EncodedMark (postcard Vec, row-wise)
 
 For each span with `len == 0` (style start), there's a corresponding mark:
 
 ```
 ┌────────────────────────────────────────────────────────────────────────────┐
-│ Column          │ Strategy       │ Description                             │
-├─────────────────┼────────────────┼─────────────────────────────────────────┤
-│ key_idx         │ (raw varint)   │ Index into keys array                   │
-│ value           │ (postcard)     │ LoroValue for the style                 │
-│ info            │ (raw u8)       │ TextStyleInfoFlag byte                  │
-└─────────────────┴────────────────┴─────────────────────────────────────────┘
+│ For each EncodedMark (row-wise postcard):                                  │
+├─────────────────┬──────────────────────────────────────────────────────────┤
+│ varint          │ key_idx - Index into keys array                          │
+│ postcard        │ value - LoroValue for the style                          │
+│ 1 byte          │ info - TextStyleInfoFlag byte                            │
+└─────────────────┴──────────────────────────────────────────────────────────┘
 
 info byte layout:
   Bit 7 (0x80): ALIVE - Style is active

docs/encoding.md

@@ -600,7 +600,7 @@ ContainerIDs are encoded as a postcard Vec of EncodedContainer structs (row-wise
 │   1 byte      │   is_root (bool)                                 │
 │   1 byte      │   kind (u8 container type)                       │
 │   LEB128      │   peer_idx (usize)                               │
-│   SLEB128     │   key_idx_or_counter (i32)                       │
+│   zigzag varint│  key_idx_or_counter (i32)                       │
 └───────────────┴──────────────────────────────────────────────────┘
 
 For root containers (is_root = true):