vt: preserve semantic wraps across resize and reattach

[takezoh]

Summary

Preserve terminal soft-wrap boundaries in the VT screen and scrollback model, then use them to reflow retained content across resizes and reattachments.

Why

Physical terminal rows are not semantic lines. Once soft-wrap provenance is discarded, a narrow terminal snapshot cannot be reconstructed correctly when a wider client attaches: retained output stays permanently wrapped at the old width.

Changes

• track soft-wrap provenance alongside visible and scrollback rows
• rebuild logical lines and reflow them when the emulator width changes
• preserve cursor position, pending-wrap state, wide cells, and truncated scrollback heads
• add Emulator.ReattachSnapshot so reattachment emits hard boundaries as CRLF while leaving soft-wrap continuations unbroken
• add regression coverage for narrow-to-wide reflow, wide cells, explicit line breaks, cursor mapping, alternate-screen snapshots, and capped scrollback

Impact

Consumers can rebuild a terminal for a new viewport without baking the previous client's width into retained history. Existing Scrollback.Line and Scrollback.Lines accessors remain available.

Checks

• go test ./... in vt/
• go test -race ./... in vt/
• go vet ./... in vt/