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code has been replaced by a from-scratch implementation: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/settings.rs

@@ -3,7 +3,8 @@ use std::path::PathBuf;
 use serde::{Deserialize, Serialize};
 
 use crate::bubble::DEFAULT_BUBBLE_SIZE;
-use crate::tray_icon::TrayIconKind;
+use crate::usage::ProviderId;
+type TrayIconKind = ProviderId;
 
 const APP_DIR_NAME: &str = "ClaudeCodeUsageBubble";
 const SETTINGS_FILE: &str = "settings.json";
@@ -38,20 +39,20 @@ pub struct BubblePositions {
 impl BubblePositions {
     pub fn get(&self, model: TrayIconKind) -> Option<(i32, i32)> {
         match model {
-            TrayIconKind::Claude => self.claude,
-            TrayIconKind::Codex => self.codex,
+            ProviderId::Claude => self.claude,
+            ProviderId::ChatGpt => self.codex,
         }
     }
     pub fn set(&mut self, model: TrayIconKind, pos: (i32, i32)) {
         match model {
-            TrayIconKind::Claude => self.claude = Some(pos),
-            TrayIconKind::Codex => self.codex = Some(pos),
+            ProviderId::Claude => self.claude = Some(pos),
+            ProviderId::ChatGpt => self.codex = Some(pos),
         }
     }
     pub fn reset(&mut self, model: TrayIconKind) {
         match model {
-            TrayIconKind::Claude => self.claude = None,
-            TrayIconKind::Codex => self.codex = None,
+            ProviderId::Claude => self.claude = None,
+            ProviderId::ChatGpt => self.codex = None,
         }
     }
     pub fn reset_all(&mut self) {