Upgrade Rust to 1.99 and manage build caches

[popduke]

Old build outputs remain after compiler changes and test runs. Cargo 1.99 does not remove these outputs.

This PR updates the Rust toolchain to 1.99.0. It updates cargo-generate to 0.25.0 and sets the fuzz toolchain to nightly-2026-10-02. It replaces the Lua integer conversion and plugin log UTF-8 recovery code with standard library methods. Integer results, log error flags, and text length limits stay the same. Hex decoders use array chunks to meet the new Clippy check.

Build cache maintenance runs before and after Rust and Java verification. When the checkout total exceeds 30 GiB, it removes unused development build outputs and generated documentation. A build can exceed 30 GiB before the next maintenance run. Repository locks and open-file checks protect active build files. Deletion checks reject external paths, symbolic links, and tracked files. The tool keeps release outputs and the last Java Plugin bundles that passed all behavior checks. A new Java cache version replaces the old version only after all behavior checks pass. The command owner stops remaining processes when the command ends. The next maintenance run removes abandoned experiments when their files are not in use.

Validation:

• macOS arm64: the full tools/verify-rust.sh check passed for the final source files.
• Linux arm64: the full check passed for the toolchain and cache changes before the two conversion changes.
• Four Lua tests and seven plugin log tests passed for the conversion changes.
• Queue fuzz tests and the quickstart passed before the two conversion changes.

We did not measure speed or disk-space improvements for the two conversion changes.