Remove Audio 3 workspace routing

[Saco93]

Summary

• remove optional Audio3 Workspace ID routing from configuration, Settings, runtime resolution, diagnostics, and documentation
• keep Regional routing limited to fixed reviewed Beijing and Singapore Streaming/Native endpoint pairs
• keep exact Custom URLs for loopback, proxies, compatibility, and testing
• simplify endpoint resolution to an infallible region/custom selection

Compatibility

• old TOML/JSON workspace_id fields remain readable through unknown-field tolerance, are ignored, and are omitted on the next save
• old schema 4 diagnostics containing workspace_configured remain readable and omit that field when reserialized
• noncanonical host pairs continue to migrate to Custom byte-for-byte
• diagnostics schema remains 4

Validation

• cargo fmt --all -- --check
• cargo check --locked --all-targets
• cargo test --locked --all-targets — 241 passed
• cargo clippy --locked --all-targets -- -D warnings
• make validate
• make validate-qml
• git diff --check

The unrelated local src/llm.rs modification is intentionally excluded from this PR.