feat(frontend): Expand/Shrink Selection via tree-sitter AST (Shift+Alt+Right/Left)

[KrisPowers]

Summary

• Adds editor.smartSelect IPC op to the C++ backend (editor_buffer.cpp) that walks the tree-sitter parse tree from the current selection upward, returning the byte range of the smallest named node that strictly contains the selection
• GpuEditor gains expandSmartSelect / shrinkSmartSelect on its imperative handle, with a history stack so each Shrink reverses the last Expand step; history clears on any regular cursor movement
• FullscreenIDE wires editor.action.smartSelect.expand and editor.action.smartSelect.shrink into the trigger handler, enabling the Edit menu items that were previously no-ops
• Shift+Alt+Right / Shift+Alt+Left are also handled directly in the GpuEditor keydown handler so the shortcuts work without going through the menu

Works for all tree-sitter-backed languages (JS, TS, TSX, Python, C, C++, Rust, Go, Java, and more). Falls back to a no-op for unsupported files with no parse tree.

Test plan

• Open a JS/TS file, place cursor inside a string literal, press Shift+Alt+Right: selects string content, then full string with quotes, then surrounding expression, then statement, then block, and so on to file root
• Shift+Alt+Left reverses each step
• Edit menu Expand Selection / Shrink Selection items produce the same result as the keyboard shortcut
• Regular cursor movement (arrow keys, click) clears the expand history so shrink becomes a no-op
• Opening a plain-text file with no tree-sitter grammar: Shift+Alt+Right is a no-op, no error
• Verify in Python, C++, and Rust files as well

Closes #71