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/context_bar.rs

@@ -0,0 +1,45 @@
+use crate::color;
+use crate::section::{RenderContext, SectionOutput};
+
+/// Render context bar at a given bar_width. Called both at initial render
+/// and during flex expansion (with wider bar_width).
+pub fn render_at_width(ctx: &RenderContext, bar_width: u16) -> Option<SectionOutput> {
+    let pct = ctx.input.context_window.as_ref()?.used_percentage?;
+    let pct_int = pct.round() as u16;
+
+    let filled = (u32::from(pct_int) * u32::from(bar_width) / 100) as usize;
+    let empty = bar_width as usize - filled;
+
+    let bar = "=".repeat(filled) + &"-".repeat(empty);
+    let raw = format!("[{bar}] {pct_int}%");
+
+    let thresh = &ctx.config.sections.context_bar.thresholds;
+    let color_code = threshold_color(pct, thresh);
+
+    let ansi = if ctx.color_enabled {
+        format!("{color_code}[{bar}] {pct_int}%{}", color::RESET)
+    } else {
+        raw.clone()
+    };
+
+    Some(SectionOutput { raw, ansi })
+}
+
+pub fn render(ctx: &RenderContext) -> Option<SectionOutput> {
+    if !ctx.config.sections.context_bar.base.enabled {
+        return None;
+    }
+    render_at_width(ctx, ctx.config.sections.context_bar.bar_width)
+}
+
+fn threshold_color(pct: f64, thresh: &crate::config::Thresholds) -> String {
+    if pct >= thresh.critical {
+        format!("{}{}", color::RED, color::BOLD)
+    } else if pct >= thresh.danger {
+        color::RED.to_string()
+    } else if pct >= thresh.warn {
+        color::YELLOW.to_string()
+    } else {
+        color::GREEN.to_string()
+    }
+}