feat: support cross-primitive callbacks / shared refs

[cassdeckard]

Summary

Design and implement support for callbacks that modify other primitives (e.g. a List item selection that updates a Table and a TextView). This requires shared references or imperative update hooks, which clash with YAML's declarative nature.

Background

Discovered while planning a tview presentation demo implementation via tviewyaml. The Table slide has a List whose items call functions that change the Table's borders, separator, selectable mode, and the code TextView's content.

Demo Usage

Table slide list items modify table and code TextView; requires shared refs; YAML is declarative.

Proposed Approach

• Option A: Add ref or name to primitives; provide template functions like updateTable(ref, ...) or setTextViewText(ref, text) that the builder registers
• Option B: Allow onSelected to receive a multi-step action that updates state consumed by other primitives (reactive)
• Option C: Document as a known limitation; complex demos may need custom Go code

Relevant Files

• config/types.go – name/ref for primitives
• builder/builder.go – register named primitives in context
• template/builtins.go – imperative update functions
• docs/tview-coverage.md – document approach or limitation