fix(deps): raise rustfft and rand version floors

[bhavyajain0810]

What

• Raises the minimum supported rustfft version from 6 to 6.4.1.
• Raises the minimum supported rand version from 0.9 to 0.9.4.
• Preserves the existing small_rng feature.
• Keeps the change limited to the affected declarations in Cargo.toml.

Why

The existing Cargo requirements use compatible version ranges:

• rustfft = "6" allows compatible 6.x releases.
• rand = "0.9" allows compatible 0.9.x releases.

Although Cargo.lock already resolves rustfft 6.4.1 and rand 0.9.4, the manifest still permits resolution to older compatible versions.

Raising the version floors ensures that dependency resolution cannot fall below the currently validated releases while still allowing future semver-compatible updates.

Closes #273

How

• Updated rustfft to 6.4.1.
• Updated rand to 0.9.4.
• Preserved the existing features and dependency configuration.
• Confirmed that Cargo.lock requires no change because it already resolves both requested versions.
• Rebased the branch onto the latest upstream/main.

The following checks passed:

• cargo metadata --locked --format-version 1
• cargo fmt --all -- --check
• cargo clippy --locked --workspace --all-targets --all-features -- -D warnings
• cargo test --locked --workspace --all-features
• RUSTDOCFLAGS="-D warnings" cargo doc --locked --workspace --no-deps --all-features
• cargo bench --locked --workspace --no-run --all-features
• cargo machete
• git diff --check

cargo deny check currently fails on the existing upstream/main lockfile because of these pre-existing advisories:

• RUSTSEC-2026-0204 for crossbeam-epoch 0.9.18
• RUSTSEC-2026-0186 for memmap2 0.9.10

This PR does not modify Cargo.lock or either affected dependency.

Checklist

• cargo test --workspace passes
• cargo clippy --workspace -- -D warnings passes
• cargo fmt --all applied
• CHANGELOG.md updated (not required; this is not a user-facing change)
• Benchmarks added or updated (not applicable; no performance-sensitive code was changed, and existing benchmarks compile successfully)

[github-actions]

PR Check Summary

Item	Value
Branch	fix/update-dependency-floors-273
Changed crates	none
Files changed	1

CI will run: build, test, clippy, fmt, cargo-deny, DCO, semver.
Reviewer assigned from CODEOWNERS.

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📒 Files selected for processing (1)

• Cargo.toml

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