fix(scripts,#1257): cargo-test.sh wrapper for platform GPU features

[joelteply]

Summary

Make `cargo test` Just Work on every platform without forcing the dev to memorize the platform-correct Cargo feature incantation.

Before (the friction this card was filed for):
```bash
cd workers/continuum-core && cargo test tick_db_handle --lib

→ fails in vendored llama crate; "metal" or "cuda" feature required

```

After:
```bash
./scripts/cargo-test.sh tick_db_handle --lib # from src/
npm run test:rust -- tick_db_handle --lib
```

The wrapper sources the existing `scripts/shared/cargo-features.sh` detector — same single source of truth that `build-with-loud-failure.sh` and `git-prepush.sh` already use — and forwards arbitrary args to `cargo test` with platform-correct features appended.

Acceptance criteria check (from #1257)

• Focused `continuum-core` Rust test command is documented and/or wrapped so macOS uses `metal,accelerate` by default. ← `scripts/cargo-test.sh` + `npm run test:rust` + `TESTING.md`.
• The fix does not weaken or remove the CPU-only compile guard. ← Wrapper just appends features; the no-CPU-fallback contract (`tests/no_cpu_fallback_contract.rs`, perf(inference): silent CPU model fallbacks violate the no-CPU-fallback alpha rule #1262) still fires when invoked directly. `CARGO_TEST_NO_FEATURES=1` escape hatch lets you reproduce the bare-command failure to verify the guard.
• Local docs distinguish intentional CPU-only test mode from normal Metal-backed local development. ← `TESTING.md` "CPU-only debug mode" section.
• Fresh-install validation includes this path — left for a follow-on if Carl/install adds a smoke step. The wrapper itself is platform-agnostic and discoverable; the install path doesn't currently run cargo test.

Files

• `scripts/cargo-test.sh` — wrapper script (new, +90 LOC)
• `package.json` — adds `test:rust` npm alias next to `test:precommit`/`test:prepush`
• `workers/continuum-core/TESTING.md` — documents the friction, the wrapper, and the per-platform feature mapping (new)

Test plan

• `./src/scripts/cargo-test.sh --test generated_barrel_sync` → 8 passed, 0 failed (auto-applied `--features metal,accelerate` on this M5 Pro).
• Precommit (TypeScript + browser ping): PASSED

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