fix(jupyter): emit Quarto-canonical ::: {.cell} cells so captures splice into previews (bd-gthycd33)

Jupyter's post-engine markdown lacked the Div.cell wrapper that
CaptureSpliceStage and the Bootstrap cell CSS key on, so executed
output never spliced into hub-client/q2-preview (knitr worked). Emit
the knitr/Q1-parity shape, retire the divergent test-only AST path
(JupyterTransform/outputs_to_blocks), and pin the contract with a
splice regression pair + a 5-case cross-engine output-parity suite.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: cscheid

claude-notes/plans/2026-07-01-bd-gthycd33-jupyter-cell-wrapper.md

@@ -41,18 +41,14 @@ mod daemon;
 mod error;
 mod execute;
 mod kernelspec;
-mod output;
 mod session;
 mod text_execute;
-mod transform;
 
 pub use daemon::{JupyterDaemon, daemon};
 pub use error::{JupyterError, Result};
 pub use execute::{CellOutput, ExecuteResult, ExecuteStatus, MimeBundle};
 pub use kernelspec::{ResolvedKernel, find_kernelspec, is_jupyter_language, list_kernelspecs};
-pub use output::{OutputOptions, outputs_to_blocks};
 pub use session::{KernelInfo, KernelSession, SessionKey};
-pub use transform::JupyterTransform;
 
 use std::path::{Path, PathBuf};
 use std::sync::OnceLock;