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/section/cost_velocity.rs

@@ -0,0 +1,19 @@
+use crate::color;
+use crate::section::{RenderContext, SectionOutput};
+
+pub fn render(ctx: &RenderContext) -> Option<SectionOutput> {
+    if !ctx.config.sections.cost_velocity.enabled {
+        return None;
+    }
+
+    let velocity = ctx.metrics.cost_velocity?;
+    let raw = format!("${velocity:.2}/min");
+
+    let ansi = if ctx.color_enabled {
+        format!("{}{raw}{}", color::DIM, color::RESET)
+    } else {
+        raw.clone()
+    };
+
+    Some(SectionOutput { raw, ansi })
+}