fix(me-pool): resolve 0-writer blackouts with zero-allocation constraints

- Converts adaptive floor logic from proactive idle drops to reactive
  global capacity constraints, fixing sudden drops to 0 active writers.
- Implements `base_req` override gateway via `can_open_writer_for_contour`,
  retaining critical connections for starved datacenters during bursts.
- Applies zero-allocation performance optimization via direct inner lock iter,
  avoiding `HashSet` generation and deep `RwLock` checks in writer validation paths.
- Scrubs now-dead variables/evaluations (`adaptive_idle_since`,
  `adaptive_recover_until`) to fulfill strict memory & hot-path constraints.

src/transport/middle_proxy/pool_writer.rs

@@ -342,7 +342,7 @@ impl MePool {
         allow_coverage_override: bool,
     ) -> Result<()> {
         if !self
-            .can_open_writer_for_contour(contour, allow_coverage_override)
+            .can_open_writer_for_contour(contour, allow_coverage_override, writer_dc)
             .await
         {
             return Err(ProxyError::Proxy(format!(