## Problem

Start() returns ErrInvalidState when the box was already started by Create(), even though the box is actually running. Affects all users following the standard pattern of calling create() then start().

## Root Cause

Create() was updated to auto-start the box, but Start() was not made idempotent. When the box is already Running, Start() returns ErrInvalidState, which propagates as an error.

## Change

Extract startIdempotent() in the runner client: when ErrInvalidState is received, call Info() to confirm the box is actually Running before suppressing the error. All other states (Stopping, etc.) propagate the error unchanged.

Unconditionally suppressing ErrInvalidState would be wrong — the error code also covers Stopping/Paused states that are genuine failures.

Also adds api_id() to BoxBackend for REST API ID handling and fixes CLI linker issue on Linux.

## Verification

```bash
cd apps/runner
go test -tags boxlite_dev ./pkg/boxlite/ -run TestStartIdempotent -v
```

All 4 tests pass.