## Problem
The edit tool's intra-line renderer can drop indentation from a removed
line when text is inserted before otherwise unchanged content.
For example, this input:
```diff
-83 await commitRecordingChunk({
+83 const result = await commitRecordingChunk({
```
was rendered as:
```diff
-83 await commitRecordingChunk({
+83 const result = await commitRecordingChunk({
```
`diffWords()` groups the shared leading spaces into the added segment
(`" const result = "`). The existing highlighting logic moved those
spaces outside inverse styling, but only appended them to the added
line.
## Solution
Extract whitespace shared by both lines before calling `diffWords()`,
then initialize both rendered lines with that prefix. This preserves
indentation without changing the existing word-level highlighting
behavior. The existing handling for whitespace that belongs to only one
side remains unchanged.
Added regression coverage for:
- insertion before indented content;
- replacement within indented content;
- keeping shared indentation outside inverse highlighting.
## Validation
- `npm run check`
- `./test.sh`
- targeted `packages/coding-agent/test/diff.test.ts` (3 tests)
