test: shared repo_root() helper for live integration tests

[Coldaine]

Context: PR #410 added two live integration tests, each with its own small `repo_root()` helper:

• `crates/coldvox-stt/tests/http_remote_live.rs` walks one parent (`crates/coldvox-stt/` → `crates/`)
• `crates/app/tests/http_remote_wiring_live.rs` walks two parents (`crates/app/` → `crates/` → repo root)

This works today but will rot if a 3rd live test lands or if the crate layout changes.

Task: Extract a shared `repo_root()` (and optionally `load_test_wav_i16(name)`) test helper. Location options:

1. New `crates/coldvox-foundation/tests/common/` module, re-exported behind a `#[cfg(feature = "test-helpers")]` flag
2. A tiny `test-utils` dev-dep-only internal crate (matches what `crates/coldvox-stt/tests/common/` seems to be moving toward)

Migrate the two existing live tests to use it.

Scope: Only worth doing when a 3rd live test shows up; defer otherwise. Leave open as a tracking issue.