fix(core): add missing requestRender() in CodeRenderable streaming content setter - #965

…ntent setter (anomalyco#965)

Fixes anomalyco#963

## Summary
When streaming content into a `CodeRenderable` with
`_drawUnstyledText=false` and an active `_filetype`, the `content`
setter was returning early without calling `requestRender()`. If the
renderer was otherwise idle (no animations, no input), no frame was ever
scheduled, so `renderSelf()` never ran, `startHighlight()` never
started, and the display stayed stale indefinitely.

## Root Cause
The streaming optimization in the `content` setter:
```ts
if (this._streaming && !this._drawUnstyledText && this._filetype) {
  return  // no requestRender()
}
```
Stored the new content and marked highlights dirty, but never scheduled
a render frame.

## Fix
Added `this.requestRender()` before the early return:
```ts
if (this._streaming && !this._drawUnstyledText && this._filetype) {
  this.requestRender()
  return
}
```

## Test
Added `packages/core/src/renderables/__tests__/Code.test.ts` with a test
that verifies:
1. Setting content in streaming mode schedules a render
2. The subsequent render triggers `startHighlight()` (asserted via
`isHighlighting` and mock client state)

## Verification
- New test: **1 pass**
- Existing `LineNumberRenderable` tests (which exercise
`CodeRenderable`): **29 pass, 0 fail**