feat: complete Rust port of claude-statusline

Port the entire 2236-line bash statusline script to Rust.
Implements all 25 sections, 3-phase layout engine (render, priority
drop, flex/justify), file-based caching with flock, 9-level terminal
width detection, trend sparklines, and deep-merge JSON config.

Release binary: 864K with LTO. Render time: <1ms warm.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/metrics.rs

@@ -0,0 +1,47 @@
+use crate::input::InputData;
+
+/// Derived metrics computed once from InputData, reused by all sections.
+#[derive(Debug, Clone, Default)]
+pub struct ComputedMetrics {
+    pub total_tokens: u64,
+    pub usage_pct: f64,
+    pub cost_velocity: Option<f64>,
+    pub token_velocity: Option<f64>,
+    pub cache_efficiency_pct: Option<f64>,
+}
+
+impl ComputedMetrics {
+    pub fn from_input(input: &InputData) -> Self {
+        let mut m = Self::default();
+
+        if let Some(ref cw) = input.context_window {
+            let input_tok = cw.total_input_tokens.unwrap_or(0);
+            let output_tok = cw.total_output_tokens.unwrap_or(0);
+            m.total_tokens = input_tok + output_tok;
+            m.usage_pct = cw.used_percentage.unwrap_or(0.0);
+
+            if let Some(ref usage) = cw.current_usage {
+                let cache_read = usage.cache_read_input_tokens.unwrap_or(0);
+                let cache_create = usage.cache_creation_input_tokens.unwrap_or(0);
+                let total_cache = cache_read + cache_create;
+                if total_cache > 0 {
+                    m.cache_efficiency_pct = Some(cache_read as f64 / total_cache as f64 * 100.0);
+                }
+            }
+        }
+
+        if let Some(ref cost) = input.cost {
+            if let (Some(cost_usd), Some(duration_ms)) =
+                (cost.total_cost_usd, cost.total_duration_ms)
+            {
+                if duration_ms > 0 {
+                    let minutes = duration_ms as f64 / 60_000.0;
+                    m.cost_velocity = Some(cost_usd / minutes);
+                    m.token_velocity = Some(m.total_tokens as f64 / minutes);
+                }
+            }
+        }
+
+        m
+    }
+}