Kill the surviving mutants in graph-layout #66

Node sizes and gaps from the grid metrics, the lane placement bookkeeping and the options passed to ELK are not asserted. These are the most-affected files in @exadev/process-graph-layout on main's mutation run (https://github.com/ExaDev/process-graph/actions/runs/38170535483), with a few of the lines a surviving or uncovered mutant changes; the shard log and the uploaded HTML report have the full list.

packages/core/graph/layout/src/metrics.ts (5 survived, 7 without coverage):

• packages/core/graph/layout/src/metrics.ts:46:10 ArithmeticOperator mutant, survived, on width: units.width * GRID,
• packages/core/graph/layout/src/metrics.ts:47:11 ArithmeticOperator mutant, survived, on height: units.height * GRID,
• packages/core/graph/layout/src/metrics.ts:54:33 ArithmeticOperator mutant, survived, on export const DEFAULT_NODE_GAP = NODE_GAP_UNITS * GRID;
• packages/core/graph/layout/src/metrics.ts:55:34 ArithmeticOperator mutant, survived, on export const DEFAULT_LAYER_GAP = LAYER_GAP_UNITS * GRID;

packages/core/graph/layout/src/layout.ts (7 survived, 3 without coverage):

• packages/core/graph/layout/src/layout.ts:157:7 ObjectLiteral mutant, survived, on { once: true, signal: stop },
• packages/core/graph/layout/src/layout.ts:218:9 CallExpression mutant, survived, on placedInLane.add(element);
• packages/core/graph/layout/src/layout.ts:225:9 ConditionalExpression mutant, survived, on if (!placedInLane.has(element.id)) {
• packages/core/graph/layout/src/layout.ts:238:24 StringLiteral mutant, survived, on edges.push({ id: `e${index}`, source, target });

packages/core/graph/layout/src/elk-engine.ts (9 survived, 0 without coverage):

• packages/core/graph/layout/src/elk-engine.ts:17:21 StringLiteral mutant, survived, on const RANDOM_SEED = "1";
• packages/core/graph/layout/src/elk-engine.ts:20:10 StringLiteral mutant, survived, on right: "RIGHT",
• packages/core/graph/layout/src/elk-engine.ts:22:9 StringLiteral mutant, survived, on left: "LEFT",
• packages/core/graph/layout/src/elk-engine.ts:23:7 StringLiteral mutant, survived, on up: "UP",

Kill them by asserting the behaviour, not by loosening or removing any existing assertion. Raise the package's breakThreshold from the new baseline once they are killed. Found while setting the break thresholds in #60.