vt: reuse scrollback line buffers

[nmelo]

Summary

Reuse scrollback line buffers instead of allocating a fresh slice on every Scrollback.Push.

This keeps the current copy semantics, but once scrollback is full it reuses the evicted line buffer when capacity fits and falls back to a sync.Pool when it does not.

Closes #821.

What changed

• replace slices.Clone-on-every-push with buffer reuse in vt/scrollback.go
• recycle dropped buffers from SetMaxLines and Clear
• add tests covering source-line alias safety and evicted-buffer reuse

Why

I profiled this through initech, which embeds x/vt for multiple panes.
Using the same 11-pane synthetic workload before and after this patch:

inuse_space

• before: 320.14 MB total heap
• after: 236.02 MB total heap
• delta: -84.12 MB (-26.3%)

alloc_space

• before: 3237.93 MB cumulative heap allocations
• after: 243.97 MB
• delta: -2993.96 MB (-92.5%)

The dominant hotspot moved from:

• before: slices.Clone at 3064.29 MB alloc_space (94.64%)
• after: Scrollback.acquireLine at 170.39 MB alloc_space (69.84%)

Validation

• go test ./... in vt
• go test ./internal/tui ./cmd in initech with a local replace to this fork
• make test in initech