[codex] Remove unused Rust helpers (#17146)

## Summary

Removes high-confidence unused Rust helper functions and exports across
`codex-tui`, `codex-shell-command`, and utility crates.

The cleanup includes dead TUI helper methods, unused
path/string/elapsed/fuzzy-match utilities, an unused Windows PowerShell
lookup helper, and the unused terminal palette version counter. This
keeps the remaining public surface smaller without changing behavior.

## Validation

- `just fmt`
- `cargo test -p codex-tui -p codex-shell-command -p codex-utils-elapsed
-p codex-utils-fuzzy-match -p codex-utils-string -p codex-utils-path`
- `just fix -p codex-tui -p codex-shell-command -p codex-utils-elapsed
-p codex-utils-fuzzy-match -p codex-utils-string -p codex-utils-path`
- `git diff --check`

codex-rs/tui/src/render/renderable.rs

@@ -200,15 +200,6 @@ impl<'a> ColumnRenderable<'a> {
     pub fn push(&mut self, child: impl Into<Box<dyn Renderable + 'a>>) {
         self.children.push(RenderableItem::Owned(child.into()));
     }
-
-    #[allow(dead_code)]
-    pub fn push_ref<R>(&mut self, child: &'a R)
-    where
-        R: Renderable + 'a,
-    {
-        self.children
-            .push(RenderableItem::Borrowed(child as &'a dyn Renderable));
-    }
 }
 
 pub struct FlexChild<'a> {
@@ -374,15 +365,6 @@ impl<'a> RowRenderable<'a> {
         self.children
             .push((width, RenderableItem::Owned(child.into())));
     }
-
-    #[allow(dead_code)]
-    pub fn push_ref<R>(&mut self, width: u16, child: &'a R)
-    where
-        R: Renderable + 'a,
-    {
-        self.children
-            .push((width, RenderableItem::Borrowed(child as &'a dyn Renderable)));
-    }
 }
 
 pub struct InsetRenderable<'a> {