E2e workspace - #13

Ports dagger/java-sdk#13: the repository's dagger.toml registered this SDK
against the e2e fixture modules and the fixture client, so the real workspace
advertised test fixtures as managed. That registration moves to a test-only
dagger.toml under the fixtures directory, reached through Fixtures.workspace.

Incomplete — pushed for discussion, not for merge.

discovery:* passes. generate:* and client:* cannot work as written, because
codegen needs the session's client-backed workspace and the fixtures config can
only be selected by a synthetic one:

  - asWorkspace(cwd:) re-runs config detection, so the nested dagger.toml is
    selected, but the workspace carries no client ID and
    ModuleSource.generateLocalDependencies (mod.dang:97, added in #12) fails in
    withWorkspaceClientContext.
  - Workspace.withWorkdir keeps the client but only clones and sets Cwd, so
    ConfigFile stays the repository's dagger.toml and currentModule.asSDK(ws)
    reports the SDK is not installed. Codegen then resolves "typescript" to the
    builtin SDK rather than this one, which passes for the wrong reason.

java-sdk does not hit this: its Mod.generate reads only introspectionSchemaJSON
and builds the generated files in its own container, so a synthetic workspace is
enough. Unblocking here needs an engine change — withWorkdir re-detecting
config, or generateLocalDependencies working on a synthetic workspace.

Signed-off-by: Tom Chauveau <tom@dagger.io>