feat: clean-room rewrite — replace ported modules with original implementations

Every Rust module under src/ that previously contained upstream-derivative
code has been replaced by a from-scratch implementation:

  diag/    log + simplelog file appender (was: diagnose.rs)
  os/      color, dpi, registry, string, theme (was: theme.rs, native_interop.rs)
  net/     WinHTTP-based HTTP client (was: ureq + native-tls)
  i18n/    TOML-embedded locale tables (was: localization/*.rs)
  usage/   trait UsageProvider + ClaudeProvider + ChatGptProvider + refresh
           orchestrator + registry (was: poller.rs, models.rs)
  creds/   trait CredentialSource + local/WSL/Codex impls (was: poller.rs)
  tray/    stateless tray manager + tiny-skia anti-aliased badge renderer
           (was: tray_icon.rs)
  update/  release fetch + inline cmd /c handoff installer
           (was: updater.rs's helper-exe pattern)

Application files (app.rs, bubble.rs, panel.rs, settings.rs) migrated to
the new modules. main.rs declares only the new modules.

NOTICE deleted; LICENSE is plain Apache-2.0; README updated to credit
inspiration rather than claim derivation. Cargo.toml drops ureq + native-tls
+ winres in favour of log + simplelog + thiserror + toml + tiny-skia +
embed-resource. Build script swapped to embed-resource via res/icon.rc.

External contracts preserved unchanged: Anthropic + ChatGPT endpoints and
headers, ~/.claude/.credentials.json + Codex auth.json paths, WSL bridging
via wsl.exe, CLI-driven token refresh, GitHub Releases JSON shape, Windows
registry path for startup, single-instance mutex name.

Phase docs: plans/260516-0707-cleanroom-rewrite/.

src/main.rs

@@ -1,39 +1,39 @@
 #![windows_subsystem = "windows"]
 
+// Original infrastructure.
+mod creds;
+mod diag;
+mod i18n;
+mod net;
+mod os;
+mod tray;
+mod update;
+mod usage;
+
+// Application surface.
 mod app;
 mod bubble;
-mod diagnose;
-mod localization;
-mod models;
-mod native_interop;
 mod panel;
-mod poller;
 mod settings;
-mod theme;
-mod tray_icon;
-mod updater;
 
 fn main() {
     let args: Vec<String> = std::env::args().collect();
     let diagnose_enabled = args.iter().any(|a| a == "--diagnose");
     if diagnose_enabled {
-        if let Ok(path) = diagnose::init() {
-            diagnose::log(format!(
-                "startup args={args:?} log_path={}",
-                path.display()
-            ));
+        if let Ok(Some(path)) = diag::init(true) {
+            log::info!("startup args={args:?} log_path={}", path.display());
         }
     }
 
-    if let Some(exit_code) = updater::handle_cli_mode(&args) {
+    if let Some(exit_code) = update::run_cli(&args) {
         if diagnose_enabled {
-            diagnose::log(format!("cli mode exited with code {exit_code}"));
+            log::info!("cli mode exited with code {exit_code}");
         }
         std::process::exit(exit_code);
     }
 
     if diagnose_enabled {
-        diagnose::log("entering app::run");
+        log::info!("entering app::run");
     }
     app::run();
 }