[Bug] useTheme returns unstable token resolver and API object

[nynexman4464]

Problem

useTheme memoizes its resolved tokens map, but creates a new token function and return object on every render:

const token = (name: string) => tokens[name] ?? '';
return {name, mode, token, tokens};

This defeats memoization in consumers. For example, useChartColors memoizes its palette from token, but the changing resolver causes it to rebuild the full color API after every unrelated rerender. An effect depending on that API reruns even though the theme and mode did not change.

Proposed fix

• memoize token from the resolved tokens map
• memoize the returned UseThemeReturn object from its exposed members
• preserve identity changes when the theme or effective mode actually changes

Regression coverage

A test rerenders useTheme with no theme change and verifies both the token resolver and return object remain stable. The test fails on current main and passes with the patch.

No public types or resolved token values change.