Deterministic Solid pod seeding

Keep server lifecycle ownership outside this package. Pass a structural SeedTarget for an
existing account or an AccountProvisioner when the layout creates accounts. Do not import a
harness or server package into application code that defines a layout.

Layout rules

• Paths are pod-root-relative and begin with /; never target .acl directly.
• Prefer { instance } for generated RDF, { dataset } for caller-owned RDF/JS data, and
  { body } only for already-issued artifacts such as JSON-LD VCs.
• Assign one generated instance to one resource per pod. Cross-instance references are rebased
  only after every assignment is known.
• Pass placeholderBase whenever synthetic-rdf used a non-default base. An unmapped placeholder
  is an error, never an absolute IRI that may leak into the pod.
• An expander represents one mutually dependent issuance or computation. Call that computation
  once and return every related resource from the same result.

Mode choice and recovery

Use create for fresh e2e pods, ensure for long-lived development pods, and replace for a
deliberate deterministic rewrite. ensure leaves an existing primary resource unchanged but
creates or rewrites its requested ACL so policy converges. It treats expander groups as all-or-none:
if every primary resource and requested ACL sidecar exists, primary resources skip while ACLs
converge; if none exists, the whole group is created; a partial set errors. create also preflights
the group and keeps If-None-Match: * on every actual write.

On SeedError, inspect error.manifest. A group marked partial can contain created or
replaced members followed by one failed member and unwritten members. Repair it by running
the exact same deterministic layout in replace mode. Do not retry only the failed member because
the group output is one consistency unit.

Access control and tests

Specify access through AccessSpec; never emit .acl Turtle yourself. The package uses
@solid/object wrappers and always preserves owner read/write/control. Test pure behavior with an
injected Fetch implementation, and keep one integration suite against the selected Solid server
outside the browser-safe core. Assert content types, conditional headers, read-back bytes, group
manifest states, and typed ACL authorization values.