feat(search)!: make path the whole statement of what projection reads

[ddeboer]

What

Make a SearchField’s path the complete statement of what the projection reads from the graph, so a schema-driven extraction generator (#548) and its contract test become possible.

Today derive closures receive the framed node and read arbitrary predicates through irisOf / literalsOf / firstLiteralOf, so path is not the whole story – in the Dataset Register’s Dataset type, 13 of 23 predicates are readable only from inside a closure. A generator that emits a CONSTRUCT from the schema would silently miss every one of them.

Changes

• derive loses its FramedNode argument – its signature is now (document) => unknown. A derive computes only from fields declared before it, never from the graph. path says what to read; derive says what to compute from what was read.
• Internal fields, consistently across every kind. A field declaring no role (output / searchable / filterable / facetable / sortable) is an internal field: projected into the in-memory document so a later derive can read it, then pruned before the document reaches a writer, and absent from the engine collection definition – not stored, not indexed, no RAM. A new shared isInternalField predicate is the single source both the projection (prunes by it) and the Typesense collection definition (omits by it) use, so they cannot disagree. Previously only zero-role text fields behaved this way; the other kinds wrote and declared unconditionally.
• The IR readers are unexported (irisOf, literalsOf, firstLiteralOf) – they are now internal to projection.
• Field-model docs (packages/search/README.md, JSDoc) and the projection / collection-definition tests are updated for the new behaviour; test fixtures that previously relied on a zero-role field being projected now declare the role they actually intend.

Breaking

Breaking for @lde/search: the public derive signature changes and the three IR readers are no longer exported. Consumers migrate their derives to internal fields plus document reads – e.g. the Dataset Register replaces irisOf(node, 'urn:dr:class') with a zero-role classes reference field whose value a derive reads off the document. That consumer migration is out of scope here (separate repo, gated on #548).

Notes

Two pre-existing, degenerate edges are left as-is rather than widening scope: a field literally named id with no role would be pruned (an id-named field already clobbered the document key before this change), and a defaultSortingField pointing at an internal field would reference an omitted collection field (a default sort field should be sortable, hence never internal). Both belong in a separate declaration-validation change.

Implements #607.