Expose a public write surface for external tooling (benchmarks need the interceptor)

[jakezwang]

The new benchmark suite (https://github.com/argon-lab/benchmarks) measures branch creation, time-travel, snapshots, and storage cost through pkg/walcli — but two planned suites are blocked because external modules cannot reach any write path:

1. Per-operation write throughput — internal/driver/wal.Interceptor has exactly the right surface (InsertOne/UpdateMany/... with real results), but internal/ is unreachable from outside and walcli.Services does not expose a constructor for it.
2. Storage amplification under divergence — creating N branches that each modify δ% of documents requires branch writes, same blocker. (ImportService.ImportDatabase takes an internal options type, so import can't be driven programmatically from outside either; the benchmark shells out to the CLI for seeding as a workaround.)

Suggested minimal surface, either works:

• walcli.Services.NewInterceptor(projectID, branchID string) (*driverwal.Interceptor, ...) returning the concrete type (callers can hold and use values of internal types; they just can't import the package), or
• a small exported wrapper in pkg/ with basic-typed signatures.

Also worth exporting an ImportOptions equivalent while at it.

Context: the benchmark repo is now the gate for every published performance number (see its README). The write-path suites land there as soon as this surface exists.

[jakezwang]

Unblocked by #20: walcli.Services.WriterFor(project, branch) returns a programmatic writer (Put/PutMany/Delete with explicit document states) usable from external modules — call methods on the returned value without importing internal packages. Import runs are covered by the existing Services.ImportDatabase wrapper.