Bytea<'fcx> zero-copy surface — the type #36 export_graph/import_graph wants

[styk-tv]

pgRDF has no bytea surface at all. Every loader entry point takes a file
path or text; the only &[u8] in the tree is internal (sniff_is_ntriples,
parser scanning helpers). There is no way to hand the extension a document that
is already in the database or already in the client's hands without routing it
through a path or a text round-trip.

pgrx 0.19.0 added Bytea<'fcx> — a bytea argument type with zero-copy return.

Why this matters to #36

#36 (pgrdf.export_graph / import_graph — native portable dump+restore, proven
at 120M) is, at the SQL level, a bytea in/out surface. Implemented over text or
via a temp file it inherits a copy per call and an encoding round-trip; at the
scale #36 cites, that is the difference between a usable primitive and a
demonstration.

Bytea<'fcx> is the type that makes the natural signature the fast one.

Scope

• Evaluate Bytea<'fcx> for the export/import signature.
• Consider a load_turtle-family overload taking bytea, which would also serve
  client-side upload paths that currently have to stage a file.

Why P2

#36 is P4 and unstarted. This is preparatory — it sharpens #36's design rather
than unblocking anything now.

Relates

Serves #36.