VcsCore's binding contract caching mechanism, specifically a performance regression where describe() calls are redundantly executed on subsequent exec calls under the "always-on carrier."

**Summary.** `VcsCore` caches resolved binding contracts per instance
(`BindingContractResolver` — `vcs-core/packages/core/src/vcs_core/_binding_contracts.py`;
constructed once per `VcsCore` in `vcscore.py`). The intended invariant: for a live binding,
`driver.describe()` + schema validation + command/ingress contract compilation run **once per
`VcsCore` lifetime**, and subsequent `exec` calls reuse the cached `ResolvedDriverBinding`.

Since #4 (portable copy carrier + cross-platform auto backend, merge `79983e4`), that invariant
is broken under the always-on carrier: two `exec` calls against one activated `VcsCore` observe
**3** `describe()` calls where **1** is expected.

**Repro** (on `main`):

```bash
uv run pytest vcs-core/packages/core/tests/unit/test_binding_surface.py::test_vcscore_exec_uses_cached_resolved_binding_contract_without_runtime_describe -q
```

Fails with `assert 3 == 1` (`driver.describe_calls`). The test body is the minimal repro: a
counting driver, one `VcsCore`, `fork` a scope, two `exec("plain", "echo", ...)` calls.

**Impact.** Functionally harmless — command results are correct — but each `exec` now pays
redundant `describe()` + full schema validation + contract compilation, and any driver whose
`describe()` is not cheap/idempotent inherits a per-exec cost the contract layer was built to
prevent. The caching invariant itself is the regression.

**Mechanism (unconfirmed — needs a bisect within #4).** The cache
(`_binding_contracts.py::BindingContract...