feat(lane-graphs): add an interpreter, so the graphs are not prose wearing a schema

[MajorLift]

Stacked on #107 — targets jongsun/add/lane-graphs, not main.

What

The three .graph.json specs name an executor and nothing parses them. A run can skip a node and still describe itself as having followed the graph, because nothing holds the description to the file — which is the failure this package is about, one level up.

What it does and does not do

Worth being exact about, since overstating it would reproduce that failure.

The check, run and expect fields are natural language. No interpreter evaluates them and this one does not pretend to — a node's verdict is supplied by whoever ran it. Routing stays judgement, as the skill's own contract says.

What is mechanised is the part that was actually being skipped:

• every node must carry a verdict
• a pass must name its evidence — a pass with an empty evidence field is a claim about a run, which is refused everywhere else in this package
• absent is a first-class outcome, reported rather than silently dropped
• a ledger entry naming no node in the graph is an error, not ignored
• a graph with an unanswered node cannot report complete

Structural validation refuses a graph missing a baseline or a prediction, those being the shape of every vacuous run: a treatment applied to nothing, or a result compared to no stated expectation.

Usage

graph-run.mjs <graph.json>                    validate structure
graph-run.mjs <graph.json> --scaffold         emit a blank ledger
graph-run.mjs <graph.json> --ledger <l.json>  check a filled-in ledger

Exit 0 = valid / every node answered and passing. Exit 1 = a node failed, is unanswered, or the ledger does not match the graph. Exit 2 = usage or parse error.

Controls

case	exit
all three graphs, structure only	0
blank ledger (every node absent)	1
fully answered with evidence	0
one node silently removed	1 — UNANSWERED baseline
pass with empty evidence	1
ledger entry naming no graph node	1

node --test test/*.test.mjs — 61/61.

One note on those controls: an earlier version of them reported three false passes, because echo "$(printf ...) exit=$?" resets $? via the command substitution before it is read. The controls were measuring printf. Worth recording, given what this script is for.