docs: describe lossless renderer markers in developer docs

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

DEVELOPER.md

@@ -148,6 +148,23 @@ already excluded them (§3).
 Glues become `(space :width (N))` display properties, so justification
 is pixel-exact in GUI Emacs and column-exact in batch/tty.
 
+The output is **lossless**: boxes are located in the source string
+(`ekp--box-offsets`), and every synthesized or hidden piece records the
+original text it stands for —
+
+| property          | on                    | value / meaning                |
+|-------------------|-----------------------|--------------------------------|
+| `ekp-glue`        | synthesized glue space| original text it replaced      |
+| `ekp-soft-break`  | inserted `\n`         | whitespace swallowed at break  |
+| `ekp-soft-hyphen` | inserted hyphen       | marker only                    |
+| `ekp-hidden`      | paragraph-edge text   | kept verbatim, `display ""`    |
+
+Zero-width glue with a non-empty original renders as the hidden
+original itself, so no character is ever dropped.  `ekp-region.el`
+inverts these four structurally (`ekp-unjustify-region`) — exact even
+after the justified text was edited — and builds
+`ekp-justify-region` / `ekp-auto-justify-mode` on top.
+
 ## 6. Looseness
 
 `ekp-looseness` ≠ 0 switches to `ekp--dp-run-loose`, a full

DEVELOPER_ZH.md

@@ -132,6 +132,21 @@ penalty/flagged 断点),主流程无 `q`/looseness(见 §6),相邻松紧惩
 Glue 渲染为 `(space :width (N))` display 属性,GUI 下像素级精确,
 batch/tty 下按字符列精确。
 
+渲染输出是**无损**的:先用 `ekp--box-offsets` 在原串中定位每个盒子,
+然后每一处合成/隐藏内容都记录它所对应的原文——
+
+| 属性              | 位置            | 值 / 含义                  |
+|-------------------|-----------------|----------------------------|
+| `ekp-glue`        | 合成的 glue 空格| 它所替换的原文             |
+| `ekp-soft-break`  | 插入的 `\n`     | 断点处被吞掉的空白         |
+| `ekp-soft-hyphen` | 插入的连字符    | 仅作标记                   |
+| `ekp-hidden`      | 段落边缘文本    | 原样保留,`display ""` 隐藏|
+
+零宽 glue 若对应非空原文,直接渲染为隐藏的原文本身,因此任何字符都
+不会丢失。`ekp-region.el` 对这四类标记做纯结构逆变换
+(`ekp-unjustify-region`)——即使排版后又被编辑过也能精确还原——并在
+其上实现 `ekp-justify-region` / `ekp-auto-justify-mode`。
+
 ## 6. Looseness
 
 `ekp-looseness` ≠ 0 时切换到 `ekp--dp-run-loose`:完整的