A chain should be able to walk a named path pattern, not only a reference field #371

Problem

A chain walks reference fields: relation takes one field or several (#211), and sibling is one more, all declared on the chain's own scheme. ADR-126 keeps it so: a chain may walk a relation only if the chain's own scheme is its sole target.

That leaves lines the record states and no chain can draw. ADR-127 names one: in a downstream record an argument runs claim → inference → claim. The step from claim to claim is two edges through another scheme, and the chain stops at the inference. ADR-127 left "chains over a derived relation" for after the logic core.

Direction (settled in discussion)

Not a field list, and not user-written clingo rules. Use named path patterns in Cypher's pattern syntax, which pass ADR-125's "rule in the line" test:

patterns:
  follows_from: (b:CLAIM)<-[:concludes]-(i:INF)<-[:premise_of]-(a:CLAIM)

• Node labels are schemes and edge types are reference fields.
• luria parses a small subset with its own lark grammar and lowers it to clingo, so the logic core stays the one engine.
• A chain walks a pattern wherever it walks a field, and named inner nodes render as "through".
• A partial match is no step. What a record calls unfinished is a pattern it writes, and rendering such a pattern is Render a named path pattern as a page listing its matches #373.

Decision: ADR-tmpfdvjo. Implementation: #372.

Out of scope (each its own decision)

• An outline view walking a pattern from a root across schemes (where OPTIONAL MATCH would earn its place).
• Transcluding document bodies so views compose into a manuscript.
• Patterns rendered as reports (Render a named path pattern as a page listing its matches #373).