`exporter.toSVG` mutates models unexpectedly

It looks like the toSVG method mutates the models passed into it, which is expected. This came up while trying to cache submodels to improve performance.

Here's a minimal repro case:

const makerjs = require("makerjs");
const opentype = require("opentype.js");

const font = opentype.loadSync("./src/fonts/ReliefSingleLineCAD-Regular.ttf");

const text = new makerjs.models.Text(font, "Hi", 12);
const model = {
	models: {
		text1: text,
		text2: text,
	},
};

const svg = makerjs.exporter.toSVG(model);
console.log(svg);

The result is this exception:

TypeError: Cannot read properties of undefined (reading 'startT')
    at getActualBezierRange (/Users/xavi/code/maker-test/server/node_modules/makerjs/dist/index.js:8591:43)
    at /Users/xavi/code/maker-test/server/node_modules/makerjs/dist/index.js:8790:37
    at Array.forEach (<anonymous>)
    at /Users/xavi/code/maker-test/server/node_modules/makerjs/dist/index.js:8785:27
    at _loop_3 (/Users/xavi/code/maker-test/server/node_modules/makerjs/dist/index.js:6054:21)
    at Object.findChains (/Users/xavi/code/maker-test/server/node_modules/makerjs/dist/index.js:6058:17)
    at BezierCurve.getBezierSeeds (/Users/xavi/code/maker-test/server/node_modules/makerjs/dist/index.js:8773:31)
    at /Users/xavi/code/maker-test/server/node_modules/makerjs/dist/index.js:6170:73
    at Array.forEach (<anonymous>)
    at swapBezierPathsWithSeeds (/Users/xavi/code/maker-test/server/node_modules/makerjs/dist/index.js:6161:21)

I'm using node v22.11.0 with makerjs@0.18.1 and opentype.js@1.3.4

The issue seems to be somewhere in here: https://github.com/microsoft/maker.js/blob/1ed796b0a94d781eb050882e30fc52086d34aada/packages/maker.js/src/core/chain.ts#L405-L453

It's not obvious to me how to fix it, so I figured I'd open an issue

The exception doesn't occur when useSvgPathOnly option is enabled, which double confirms that the issue is related to the findChains function

Workaround

// Clone submodels
const model = {
	models: {
		text1: makerjs.cloneObject(text),
		text2: makerjs.cloneObject(text),
	},
};

This does works, but it add a bit of overhead. Also, it's surprising that an exporter mutates the underlying model

Start with exporter.toSVG and the chain handling in packages/maker.js/src/core/chain.ts, especially lines 405-453 and findChains. Run the shared-submodel repro from the issue, then compare it with useSvgPathOnly enabled. Done means exporting a model that references the same submodel twice no longer mutates it or throws the reported startT exception.