[labs/router] getTailGroup selects the wrong tail: unanchored digit regex and lexicographic group ordering

Which package(s) are affected?

@lit-labs/router

Description

getTailGroup in packages/labs/router/src/routes.ts picks the wrong wildcard tail in two cases:

1. Named params containing a digit hijack the tail. For a route like /user/:id1/* matching /user/42/rest/path, URLPattern produces groups {"0": "rest/path", "id1": "42"}. The check /\d+/.test(key) is unanchored, so id1 qualifies as a tail candidate, and the child route receives "42" instead of "rest/path".
2. Numeric group ordering is lexicographic. With 10+ numeric groups, key > tailKey compares strings, so group "9" wins over "10" and the tail is off by one.

Both bugs live in a single condition: https://github.com/lit/lit/blob/main/packages/labs/router/src/routes.ts#L295-L303

Reproduction

Node 18+ (native URLPattern):

const g1 = new URLPattern({pathname: '/user/:id1/*'}).exec({pathname: '/user/42/rest/path'}).pathname.groups;
// -> {"0": "rest/path", "id1": "42"}
const g2 = new URLPattern({pathname: '/*/*/*/*/*/*/*/*/*/*/*'}).exec({pathname: '/a/b/c/d/e/f/g/h/i/j/k'}).pathname.groups;

const current = (groups) => { let t; for (const k of Object.keys(groups)) { if (/\d+/.test(k) && (t === undefined || k > t)) t = k; } return t && groups[t]; };
current(g1); // -> "42", expected "rest/path"
current(g2); // -> "j", expected "k"

Through the public API: a parent route /user/:id1/* with a child route :a/:b throws No route found on goto('/user/42/rest/path') because the child receives "42" instead of "rest/path".

Workaround

Avoid digits in named param names when combined with a trailing wildcard, and avoid patterns producing 10+ numeric groups.

Is this a regression?

No. Present since the initial implementation (#2331); the function has not changed since.

Affected versions

@lit-labs/router 0.1.4 (current main)

Browser/OS/Node environment

All (logic bug, verified on Node 24 + native URLPattern).

Proposed fix

Anchor the regex (/^\d+$/) and compare group keys numerically. Regression tests via nested routes. Happy to send a PR.