fix: stop hiding user f:-namespace data from variable-predicate scans

[bplatz]

Stacked on #1593 — contains that branch's commit; diff shown here is just this change.

Problem

A ?s ?p ?o scan of a stored f:AccessPolicy node returned only its rdf:type triple — f:action, f:allow, f:exMessage were invisible, while explicit-predicate lookups of the same triples returned them. Variable-predicate scans hid the entire https://ns.flur.ee/db# namespace in the default graph.

That hide is a fossil from before named graphs existed, when commit metadata was stored in the main graph. Today commit metadata routes to the txn-meta graph (g_id=1) on every path — novelty-side stamping (stamp_graph_on_commit_flakes, all three call sites) and both indexer emitters hard-code g_id=1 — and an empirical dump of a current ledger's default graph with includeSystemFacts: true contains zero system-written f: triples. The only data the hide still caught was user-authored f: vocabulary, policies being the prime case: a wildcard inspection of a policy write looks like the write silently dropped everything but @type, and an export-by-crawl silently loses policy definitions.

Worse, the hide only ran on the indexed scan path — the novelty path never applied it — so the same query returned all triples right after the write and dropped them once the background indexer caught up.

Fix

Narrow the filter to the seven f:reifies* predicates, hidden in every graph as before. Those stay hidden deliberately: they are the system-written storage encoding of edge annotations (user transactions cannot assert them, so surfacing them would break export/re-import round-trips), they are redundant with the edge and annotation content already visible in results, and they live in the same graph as the edge they reify so there is no system graph to banish them to. opts.includeSystemFacts: true still reveals them for inspection.

Applied at all three mirrors so scan and fast-path semantics agree:

• BinaryScanOperator::is_internal_predicate (binary_scan.rs)
• the whole-graph fold's overlay-flake walk (fast_whole_graph_agg.rs)
• the fold's exactness preflight graph_has_scan_hidden_predicates (fast_whole_graph_agg.rs)

Other FLUREE_DB checks in the query crate (planner selectivity, annotation probes, wildcard hydration, fulltext datatype) were audited and are reifies-specific or unrelated — untouched.

Compatibility note

A ledger whose index predates txn-meta routing and has never been rebuilt could re-expose old commit metadata in default-graph wildcard dumps; reindexing re-emits commit metadata into the txn-meta graph.

Testing

• New it_query_wildcard_system_facts.rs: user f: data visible to wildcards pre-index and post-index (pinning the novelty/indexed flip), and a full default-graph dump stays free of commit-metadata predicates.
• Existing it_edge_annotations.rs reifies-hide and includeSystemFacts tests pass unchanged.
• Full fluree-db-api suite green (30 targets, 3,041 tests); fluree-db-query full tests green; clippy --all-features clean on both crates.
• Docs: variable-predicate visibility rule, the f:reifies* exception, and includeSystemFacts documented in docs/query/jsonld-query.md (the flag was previously undocumented).

[zonotope]

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