TUI debug panel: only 'local: args' is expandable; new variables created mid-session aren't

[JDonaghy]

Repro

1. Start a debug session in TUI (`vimcode --tui`, F5).
2. Step over the function entry (so DAP reports a local scope with multiple vars).
3. In the variables tree, observe that `local: args` has an expand chevron and can be opened to show fields.
4. Step over more lines that introduce additional locals (e.g. `let debug_log = ...`, `let skip_args = ...`).
5. Try to expand any of the newly-created variables.

Expected: any aggregate / non-primitive variable should show an expand chevron and reveal child fields when activated.

Actual: newly-introduced variables show no chevron and don't expand. Only the original `args` variable is expandable across the entire session.

Suspected cause

Likely one of:

• DAP `variablesReference` is non-zero only for the initially-requested variables; subsequent locals get returned without it.
• Engine cache (`Engine::dap_variables`) is keyed on the first scope_id and not refreshed when new locals enter scope.
• The lazy-load logic only fires for the row the user EXPANDED at session start; new rows have no expand state slot.

Files

• `src/core/dap.rs` — DAP variable response parsing
• `src/core/dap_manager.rs` — variable lazy-load + caching
• `src/core/engine/dap_ops.rs` — `Engine::dap_variables`, scope/var management
• `src/render.rs::build_var_tree` — render-side tree construction

Reference

Memory note (Session 95) mentions the build_var_tree includes group headers; possibly the chevron is only rendered for entries with non-empty children, and DAP's lazy-loaded variables show empty children until expanded.

Priority

Medium — debugging is much less useful when only one variable per session is inspectable.

[JDonaghy]

Implementation notes (2026-05-17, coordinator)

In the TUI debug panel, only the 'local: args' variable group is expandable. New variables created during a debug session don't appear or appear but can't be expanded.

Likely cause: The debug sidebar variable tree is built in src/render.rs (build_var_tree) or src/core/engine/ debug state. Check whether new variables from DapVariable responses are being added to the tree model after the initial stop event. The dap_variables / dap_scope_groups fields may not be refreshing on subsequent stops.

Files: src/core/engine/ (debug state management), src/core/dap.rs or src/core/dap_manager.rs (variable fetch), possibly src/render.rs (tree building — read only to understand structure).

Do NOT touch: src/gtk/, src/tui_main/mouse.rs.