Parent

Part of #1402.

Goal

Establish the directory skeleton and a CI lint that enforces "no tauri::* imports under core/" — without moving any code yet.

Scope

• Add mod core; and mod app; (empty modules) under src-tauri/src/.
• Add a CI step that fails if any file under src-tauri/src/core/ contains use tauri:: or tauri:: references.
  • Implementation: a grep-based step in .github/workflows/ci.yml, or a cargo xtask task — whichever is lighter.
  • The crate-split alternative (workspace + [lints] tauri = { level = "deny" }) is deferred — see the parent's Open Questions.
• No code is moved. Existing modules remain in place.

Out of Scope

• Any code relocation (Phases 2–6).
• Cargo workspace crate split.

Definition of Done

• New empty core/ and app/ modules compile.
• CI fails when a stray use tauri::*; is added under core/ (verify with a deliberate failing commit on a draft branch).
• cargo tauri-fmt, cargo tauri-lint, cargo tauri-test pass.