Centralize cross-platform storage paths

[brxs]

Part of #107.

What changed

• adds a host-owned AppPaths contract with distinct config, durable-data, cache, downloaded-asset, and staging roots
• preserves current macOS locations and makes existing LSDJai/Documents migrations restart-safe
• uses shallow non-roaming LocalAppData paths on Windows and XDG roots with standard fallbacks on Linux
• passes the resolved roots explicitly from Rust to Python services
• centralizes Unix and Windows virtual-environment interpreter layouts
• migrates settings, MCP, model, Stable Audio, LoRA, generated-song, and generated-sample consumers to the shared contract
• documents the platform mapping and adds spaces/Unicode/migration coverage

Why

The app and Python services previously reconstructed macOS-specific locations independently. Linux and Windows support need one authoritative path policy so large assets, settings, caches, staging, and child-service paths cannot drift.

Validation

• backend: 216 tests passed
• Rust shell: 233 passed, 1 ignored
• focused Rust path suite: 8 passed
• focused Python path/SA3/LoRA suite: 22 passed
• cargo clippy --workspace --all-targets -- -D warnings
• git diff --check

This PR intentionally excludes process supervision, secure runtime installation, audio sample formats, platform packaging, and release CI.

[brxs]

Superseded by and fully consolidated into #131. The combined branch includes this lane's substantive changes, subsequent integration fixes, and exact-head cross-platform validation. Please continue review in #131.