Fix UI lock when scrolling up during streaming output

[yogthos]

Reported symptom

Scrolling up while the agent streams responses makes the UI "stuck at wherever you scrolled up," especially when mouse-selecting text — the selection rectangle drifts away from the cursor as new content arrives.

Root cause

`scroll_offset` is "lines from the bottom edge," a fixed quantity. The viewport start is computed as:

```rust
start = total - scroll_offset - visible
```

As streaming tokens push new lines into the buffer, `total` grows but `scroll_offset` stays the same → `start` drifts forward. The user's view silently follows the bottom at the fixed offset instead of staying on the absolute content they scrolled to. Selection indices are absolute (correct), but the highlighted rows drift along with the viewport, so mouse-drags select the wrong lines.

Fix

When content is appended to the buffer while `scroll_offset > 0`, bump `scroll_offset` by the number of lines added (clamped to `max_offset`). This keeps `start` anchored to the same absolute index. When `scroll_offset == 0` the view continues to follow the bottom — that path is unchanged.

• All `buffer.push` sites in `commit_partial` / `write_line` / `write` go through a new private `push_buffer_line` helper.
• `replace_from` (used by the streaming-token markdown re-render) shifts `scroll_offset` by the size delta on each rewrite to preserve the anchor.

Test coverage (6 new tests)

• View stays anchored through 8 token appends while scrolled up.
• `replace_from` preserves the anchor on both grow and shrink rewrites.
• Bottom-anchored view (`scroll_offset == 0`) still follows new content.
• Selection indices remain absolute and unchanged under streaming appends.
• `push_buffer_line` clamps `scroll_offset` to max on tiny buffers.
• `commit_partial` routes through the anchor-aware push path.

Test plan

• `cargo build` clean
• `cargo test --bin dirge -- --skip plugin` → 272 passed, 0 failed
• Manual: while the agent is mid-stream, scroll up with mouse wheel — content stays put. Mouse-drag a selection across earlier lines — highlight stays where the cursor went.