chain table round 6: the thread bracket law (owner refinement)

The tree threads no longer touch their arrows. Each thread is
BRACKETED by checkbox bottom edges (live-measured anchors):
- starts at its parent row's checkbox bottom edge — a visible gap
  below the arrow's stroke (the stroke ends ~2.4px above the anchor)
- terminates exactly at the LAST child row's checkbox bottom edge
  (cluster: top-[23px]/bottom-[8px] incl. the 1px group rule; chain:
  top-[22px]/bottom-[8px]). Verified live: 23=23 top anchor, bottom
  ends within 0.13px of the last checkbox at both levels.

components/chain-table.tsx

@@ -322,15 +322,17 @@ export function ChainTableBody({
 							// top (header) and bottom. Never on individual rows.
 							className="relative border-b border-border/60"
 						>
-							{/* The TREE THREAD (owner round 5, trial): a vertical guide
+							{/* The TREE THREAD (owner rounds 5–6): a vertical guide at the
-							    dropping from the cluster's expander arrow down through
+							    expander arrow's column (center 37px: pl-3 + checkbox 14 +
-							    its chain rows — the arrow's column (center 37px: pl-3 +
+							    gap 4 + half the 14px expander), BRACKETED by checkbox
-							    checkbox 14 + gap 4 + half the 14px expander), from the
+							    bottom edges — it starts at the cluster row's own checkbox
-							    arrow's own height to the last row's middle. */}
+							    bottom (top-[23px], leaving the arrow's stroke a visible
+							    gap, live-measured) and terminates exactly at the LAST
+							    checkbox bottom (bottom-[8px] = 9px from the border box minus the 1px group rule, measured). */}
 							{open && group.chains.length > 0 && (
 								<div
 									data-testid={`cluster-thread-${group.key}`}
-									className="pointer-events-none absolute bottom-3.5 left-[37px] top-4 w-px bg-border/50"
+									className="pointer-events-none absolute bottom-[8px] left-[37px] top-[23px] w-px bg-border/50"
 								/>
 							)}
 							{/* The cluster row: birth-session group header. */}
@@ -368,13 +370,15 @@ export function ChainTableBody({
 									const selectedInChain = memberIds.filter((id) => selection.has(id)).length;
 									return (
 										<div key={chain.id} data-testid={`chain-block-${chain.id}`} className="relative">
-												{/* The chain's own thread: from its expander arrow down
+												{/* The chain's own thread (owner rounds 5–6): the same
-												    through the statement rows (center 73px: pl-12 +
+												    bracket law one level down — at the chain arrow's column
-												    checkbox 14 + gap 4 + half the expander). */}
+												    (center 73px), from the chain row's checkbox bottom
+												    (top-[22px]) to the LAST statement row's checkbox bottom
+												    (bottom-[8px]) — live-measured anchors. */}
 												{chainOpen && memberIds.length > 0 && (
 													<div
 														data-testid={`chain-thread-${chain.id}`}
-														className="pointer-events-none absolute bottom-3.5 left-[73px] top-3.5 w-px bg-border/50"
+														className="pointer-events-none absolute bottom-[8px] left-[73px] top-[22px] w-px bg-border/50"
 													/>
 												)}
 											<div

tests/render-chain-table.test.tsx

@@ -154,6 +154,11 @@ describe("ChainTableBody — THE shared TopBar + the lines law (owner, round 2)"
 		// the columns: the cluster arrow sits at 37px, the chain arrow at 73px
 		assert.ok(open.includes("left-[37px]"), "thread at the cluster arrow's column");
 		assert.ok(open.includes("left-[73px]"), "thread at the chain arrow's column");
+		// the BRACKET LAW (owner round 6): each thread runs from its parent
+		// row's checkbox bottom edge (a visible gap below the arrow's stroke)
+		// to the LAST child row's checkbox bottom edge — live-measured anchors
+		assert.ok(open.includes("bottom-[8px] left-[37px] top-[23px]"), "cluster thread bracketed by checkbox bottoms");
+		assert.ok(open.includes("top-[22px]") && open.includes("bottom-[8px]"), "chain thread bracketed by checkbox bottoms");
 	});
 });