Adopt quadraui's shared preview-tab tier; delete vimcode's private copy

[JDonaghy]

What

Replace vimcode's private preview/pin implementation with quadraui's shared
WorkspaceController preview tier (quadraui #597), and delete the local copy.

Why now

quadraui #597 ports vimcode's own preview policy into the framework so coord-tui can use the
same behaviour. Until this issue lands there are two implementations of the same rules — the
original here and the port there — and they drift on the first change.

That defeats the reason the port happened. The stated goal is that vimcode and coord-tui behave the
same, and that a future move toward VS Code's narrower promotion rules is one change both apps
inherit. That is only true once vimcode consumes the shared tier rather than paralleling it.

Behaviour must not change in this issue. All six promotion triggers stay exactly as they are today,
including promote-on-select (goto_tab), which is vimcode's deliberate divergence from VS Code
and is now the agreed cross-app contract.

Design

• Take the quadraui pin to a rev containing #480: TUI: Migrate Source Control panel (commit row, branch picker, help dialog) to shared primitives #597 (its own reviewable commit).
• Replace Engine::preview_buffer_id + promote_preview() + the find-else-replace branch in
  open_file_preview with calls into the shared tier.
• Map vimcode's BufferId to the controller's opaque document id.
• Keep every existing preview test green, unchanged. If a test needs editing to compile, that is a
  signal the port changed behaviour — stop and reconcile rather than adjusting the assertion.

Acceptance criteria

• preview_buffer_id and the local promote_preview no longer exist in src/core/.
• All five existing preview tests pass untouched in their assertions:
  test_open_file_preview_creates_preview_tab, test_double_click_promotes_preview,
  test_open_file_preview_double_click_promotes, test_edit_promotes_preview,
  test_save_promotes_preview, plus test_behavior_goto_tab_promotes_preview.
• Single-click in the file explorer still replaces the preview tab rather than adding one.
• Selecting a preview tab still promotes it.
• cargo build && cargo test EXIT=0.

Files

• src/core/engine/buffers.rs
• src/core/engine/windows.rs
• src/core/buffer_manager.rs
• Cargo.toml (quadraui pin bump, separate commit)

Depends on

quadraui #597 (preview tier), which depends on quadraui #596 (WorkspaceController).

Out of scope

• Any behaviour change. This is a de-duplication, not a redesign.
• The other three quadraui-duplication cleanups, filed separately.

[JDonaghy]

Coordinator Assignment

Machine: precision
Repo: vimcode
Files: (unspecified)
Do not touch: (nothing else in flight)

Briefing

Issue #658: Adopt quadraui's shared preview-tab tier; delete vimcode's private copy

What

Replace vimcode's private preview/pin implementation with quadraui's shared
WorkspaceController preview tier (quadraui #597), and delete the local copy.

Why now

quadraui #597 ports vimcode's own preview policy into the framework so coord-tui can use the
same behaviour. Until this issue lands there are two implementations of the same rules — the
original here and the port there — and they drift on the first change.

That defeats the reason the port happened. The stated goal is that vimcode and coord-tui behave the
same, and that a future move toward VS Code's narrower promotion rules is one change both apps
inherit. That is only true once vimcode consumes the shared tier rather than paralleling it.

Behaviour must not change in this issue. All six promotion triggers stay exactly as they are today,
including promote-on-select (goto_tab), which is vimcode's deliberate divergence from VS Code
and is now the agreed cross-app contract.

Design

• Take the quadraui pin to a rev containing #480: TUI: Migrate Source Control panel (commit row, branch picker, help dialog) to shared primitives #597 (its own reviewable commit).
• Replace Engine::preview_buffer_id + promote_preview() + the find-else-replace branch in
  open_file_preview with calls into the shared tier.
• Map vimcode's BufferId to the controller's opaque document id.
• Keep every existing preview test green, unchanged. If a test needs editing to compile, that is a
  signal the port changed behaviour — stop and reconcile rather than adjusting the assertion.

Acceptance criteria

• preview_buffer_id and the local promote_preview no longer exist in src/core/.
• All five existing preview tests pass untouched in their assertions:
  test_open_file_preview_creates_preview_tab, test_double_click_promotes_preview,
  test_open_file_preview_double_click_promotes, test_edit_promotes_preview,
  test_save_promotes_preview, plus test_behavior_goto_tab_promotes_preview.
• Single-click in the file explorer still replaces the preview tab rather than adding one.
• Selecting a preview tab still promotes it.
• cargo build && cargo test EXIT=0.

Files

• src/core/engine/buffers.rs
• src/core/engine/windows.rs
• src/core/buffer_manager.rs
• Cargo.toml (quadraui pin bump, separate commit)

Depends on

quadraui #597 (preview tier), which depends on quadraui #596 (WorkspaceController).

Out of scope

• Any behaviour change. This is a de-duplication, not a redesign.
• The other three quadraui-duplication cleanups, filed separately.

[JDonaghy]

Coordinator: Assignment Complete

Machine: precision
Status: done
Exit code: 0
Duration: 16m 13s
Log: /home/john/.coord/logs/c168de8b45f3.log

[JDonaghy]

Coordinator: Assignment Complete

Machine: precision
Status: done
Exit code: 0
Duration: 2m 13s
Log: /home/john/.coord/logs/6c4d7d8ac877.log