Parent

Part of #1402.

Goal

Convert the repo into a Cargo workspace and add a root-level core/ crate (Tauri-independent). Establishing the workspace boundary is what enforces "no tauri::* under core/" — at compile time, via the Cargo dependency graph, with no CI grep needed.

This issue replaces #1403; see #1402 for the reasoning behind the switch from src-tauri/src/core/ to a workspace crate.

Scope

• Add a workspace Cargo.toml at the repo root with members core and src-tauri.
• Create core/ crate at the repo root:
  • core/Cargo.toml — does not list tauri as a dependency. Package name TBD in this PR (e.g. hwviz-core to avoid shadowing the standard core crate).
  • core/src/lib.rs — empty pub mod placeholders for collector, persistence, monitoring, event_bus, settings, platform, infrastructure, models. No code is moved yet.
• src-tauri/Cargo.toml adds the new crate as a path dependency (<core-crate> = { path = "../core" }).
• Add mod app; (empty) under src-tauri/src/.
• Update tooling that assumed src-tauri/ was the cargo root:
  • src-tauri/.cargo/config.toml aliases (tauri-fmt, tauri-lint, tauri-test) extended to --workspace where appropriate, or relocated to a workspace-level config.
  • .github/workflows/ci.yml — fmt / clippy / test now run against the workspace.
• No code is moved. Existing modules under src-tauri/src/ stay in place.

Out of Scope

• Any code relocation (Phases 2–6).
• Splitting core into multiple sub-crates.
• A separate bin target for a future daemon (Non-Goal of Epic: split backend into Tauri-independent Core and thin App adapters #1402).

Definition of Done

• cargo build, cargo test --workspace, cargo clippy --workspace -- -D warnings succeed at the repo root.
• Adding use tauri::Manager; anywhere under core/src/ fails to compile. Verified manually on a draft branch — no CI grep needed.
• cargo tauri-fmt, cargo tauri-lint, cargo tauri-test continue to pass.
• npm run lint, npm run test unchanged.
• tauri dev / tauri build continue to work end-to-end on at least one host platform.