chore: pin Rust to nightly-2026-09-15

[dpotapov]

Summary

Pin local development, CI, and release builds to nightly-2026-09-15 (Rust 1.100.0-nightly). This includes rust-lang/rust#162240, which removes the extra completed incremental-cache snapshot after recompilation. An isolated development-build comparison against Rust 1.96.1 measured 17.8% less total CLI build output and 33.1% less incremental-cache space; those are whole-toolchain comparisons, not an isolated measurement of that patch.

Apply four equivalent expression simplifications required by the newer Clippy. Keep direct emptiness assertions by explicitly allowing the new assert_is_empty style lint. The declared Rust 1.88 minimum remains unchanged.

Validation

Passed locally on Apple Silicon macOS with the pinned nightly:

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo build --workspace --locked
• cargo test --workspace --locked
• cargo build --release --locked -p orc-cli
• Parsed both workflow files and verified all toolchain-install steps use the same dated nightly.

Local checks disabled the machine's sccache wrapper and custom LLVM linker, using Apple's linker instead. The workflow matrix will validate Linux, macOS, and Windows.