physics: imp nodes get the MIDPOINT depth anchor between premises and conclusion

Owner ruling (2026-09-30, live-testing loop — supersedes 02a58db's
exemption): implication nodes ARE depth-anchored after all. Their anchor
is the average of the HIGHEST premise row and the conclusion's row —
depth(imp) = (max(premise depths) + depth(conclusion)) / 2 — so the
junction always sits strictly above every premise and strictly below
the conclusion (a conclusion is >= highest premise + 1 in the logical
graph). Uniform across modes: a half-row in vertical, a ring midway in
radial (owner confirmed both).

Implementation:
- layout.ts: new ImpLink type; SimOptions/incrementalLayout gain
  impLinks (the derived chain linkage). The rank map is now computed
  once alongside the orphan mask: proposition ranks via Tarjan over the
  logical edges, then each supplied link adds its midpoint entry. An
  imp node WITH an anchor is structured (leaves the orphan mask —
  gravity projections treat it like any ranked node); WITHOUT supplied
  links, arg_ ids keep the legacy 02a58db exemption (honest fallback
  for callers that predate the field). The depth force reuses the
  precomputed map — one pass, shared.
- deriveView (core/graph.ts) emits impLinks (one per multi-premise
  live chain — single-premise chains mint no imp node); GraphSnapshot
  carries them; graph-view maps them into CanvasData; settleGraphLayout
  and replay-layout forward them (replay activates a junction's link
  exactly when its chain is delivered).

Tests (+3, 1 rewritten): the 02a58db exemption pin became the MIDPOINT
pin (premises row 0, conclusion row -500, junction pinned to -250);
mixed-depth pin (premises at 0 and 1, conclusion 2, junction at the 1.5
half-row — highest premise, not average-of-premises); legacy fallback
pin (no impLinks = never depth-anchored); deriveView impLinks emission
pin (multi-premise only). The horizontal-pole and ORPHAN RULE pins from
5554c3a survive unchanged — proposition orphans are untouched.

tests/graph.test.ts

@@ -1079,4 +1079,12 @@ describe("deriveView (the argument's geometry)", () => {
 		// Proposition ids only — no arg_ endpoint ever rides the logical set.
 		assert.ok(view.logicalEdges.every((e) => !e.from.startsWith("arg_") && !e.to.startsWith("arg_")));
 	});
+
+	it("impLinks: one linkage per MULTI-premise chain — the depth pass's midpoint input", () => {
+		const view = deriveView({
+			nodes: [prop("a"), prop("b"), prop("c")],
+			chains: [chain("arg_ab", ["a", "b"], "c"), chain("arg_ac", ["a"], "c")],
+		});
+		assert.deepEqual(view.impLinks, [{ id: "arg_ab", premises: ["a", "b"], conclusion: "c" }]);
+	});
 });