Add a uv cache size command (#16032)

## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Implement `uv cache size` to output the cache directory size in raw
bytes by default, with a `--human` option for human-readable output.

close #15821

<!-- How was it tested? -->

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/uv-cli/src/lib.rs

@@ -757,7 +757,6 @@ pub enum CacheCommand {
     Prune(PruneArgs),
     /// Show the cache directory.
     ///
-    ///
     /// By default, the cache is stored in `$XDG_CACHE_HOME/uv` or `$HOME/.cache/uv` on Unix and
     /// `%LOCALAPPDATA%\uv\cache` on Windows.
     ///
@@ -770,6 +769,12 @@ pub enum CacheCommand {
     /// Note that it is important for performance for the cache directory to be located on the same
     /// file system as the Python environment uv is operating on.
     Dir,
+    /// Show the cache size.
+    ///
+    /// Displays the total size of the cache directory. This includes all downloaded and built
+    /// wheels, source distributions, and other cached data. By default, outputs the size in raw
+    /// bytes; use `--human` for human-readable output.
+    Size(SizeArgs),
 }
 
 #[derive(Args, Debug)]
@@ -811,6 +816,13 @@ pub struct PruneArgs {
     pub force: bool,
 }
 
+#[derive(Args, Debug)]
+pub struct SizeArgs {
+    /// Display the cache size in human-readable format (e.g., `1.2 GiB` instead of raw bytes).
+    #[arg(long = "human", short = 'H', alias = "human-readable")]
+    pub human: bool,
+}
+
 #[derive(Args)]
 pub struct PipNamespace {
     #[command(subcommand)]