build: bump Rust version to 1.94.0

[velocitysystems]

Summary

Bumps the project's Rust version to 1.94.0 to match the updated Silvermine standard (silvermine/silvermine-info#47), plus dependency-spec cleanup for the example app.

Toolchain bump

• rust-toolchain.toml channel → 1.94.0
• rust-version in the workspace Cargo.toml and the example app
• CI workflow toolchain (.github/workflows/ci.yml)
• Example README Rust requirement note (1.89+ → 1.94+)

Example app dependency alignment

The example app was the only one of the Silvermine Tauri plugin examples that pinned dependencies with the = comparison operator (tauri, serde, tauri-build) and did not track a Cargo.lock. Because tauri's own runtime crates use caret ranges, a fresh resolve drifted to incompatible versions (tauri 2.10.3 against tauri-runtime 2.11.2) and pulled bitflags 2.12.0, which overflows the macro recursion limit in dispatch2.

• Dropped the redundant = operators to match the sibling examples and the Silvermine standard (which forbids comparison requirements).
• Committed a coherent Cargo.lock so the example builds reproducibly, as the audio and connectivity examples already do.

Verification

• npm run standards passes (markdownlint, clippy, fmt, eslint, commitlint, node check).
• The workspace and the example's full dependency stack compile cleanly under 1.94.0.