spike(rust/scaffold): create spike/audio-sync Cargo binary crate

[natashaannn]

User story

As a coding agent implementing the Rust FFT spike (tracked in #93), I want a minimal, buildable Cargo binary crate at spike/audio-sync/, so that subsequent issues have a clean project skeleton to add code into.

Background

RFC 0001 (#90) proposes rewriting the editing pipeline in Rust. To validate that the agent-driven Rust dev loop is workable before committing, the team is running a spike: port the FFT cross-correlation algorithm from scripts/sync/AudioSyncer.js to a standalone Rust binary and verify output matches across three target machines.

This issue creates the Cargo crate skeleton only — no algorithm logic. It must be entirely isolated from the existing Next.js/Node repo: no root-level Cargo.toml workspace, no changes to package.json, no CI config. The crate lives under spike/ and can be deleted wholesale when the spike is done.

Acceptance criteria

Happy path

Given the repository is checked out on a machine with Rust toolchain installed (rustup + stable toolchain)
When cargo build --manifest-path spike/audio-sync/Cargo.toml is run
Then it exits 0 and produces a binary

Error path / edge case

Given someone runs npm install or npm run build in the repo root
When Node resolves workspace packages
Then there is no change in behaviour — the Cargo crate is invisible to the Node toolchain

Out of scope

• Any algorithm logic — src/lib.rs and src/main.rs should be stubs only
• CI configuration or cross-compilation setup
• A root-level Cargo.toml workspace — explicitly forbidden

Technical context

Required dependencies (add to spike/audio-sync/Cargo.toml):

• rustfft = "6" — pure-Rust FFT, no native deps, deterministic across platforms. This is the only FFT crate permitted for this spike (hard constraint from spike(rust): port AudioSyncer FFT correlation to Rust to validate agent-driven dev loop #93).
• hound = "3" — WAV file reader/writer in pure Rust.

Stub src/main.rs:

fn main() {
    println!("audio-sync spike — not yet implemented");
}

Stub src/lib.rs: empty file (subsequent issues will add functions here).

.gitignore addition: add spike/audio-sync/target/ to the repo's root .gitignore so build artifacts are not tracked.

No workspace: spike/audio-sync/Cargo.toml must be a standalone manifest, not a member of any workspace. Do not create a spike/Cargo.toml.

Implementation details

1. Create spike/audio-sync/Cargo.toml:

[package]
name = "audio-sync"
version = "0.1.0"
edition = "2021"

[dependencies]
rustfft = "6"
hound = "3"

2. Create spike/audio-sync/src/main.rs with the stub above.
3. Create spike/audio-sync/src/lib.rs as an empty file.
4. Add spike/audio-sync/target/ to the root .gitignore.
5. Run cargo build --manifest-path spike/audio-sync/Cargo.toml and confirm it exits 0.
6. Commit spike/audio-sync/Cargo.toml, spike/audio-sync/Cargo.lock, spike/audio-sync/src/main.rs, spike/audio-sync/src/lib.rs, and the .gitignore change.

Additional test scenarios

• N/A — this issue scaffolds the project; no logic to test yet

Hard constraints

• Must not create a root-level Cargo.toml or any workspace config
• Must not modify any file outside spike/ except the root .gitignore
• Must use rustfft (not a C FFT binding) — the spike's purpose is to validate pure-Rust development

Dependency issues

Independent of #94 (fixtures) — can be done in parallel.
Issues #96, #97, #98 (algorithm ports) all depend on this one.