Support locked named variable-font instances

[kavya-oai]

What changed

• allow FontLibrary.use() sources to select a one-based OpenType fvar named instance
• pass that selection through Skia's collection-index convention
• keep explicitly selected instances locked instead of re-instancing them from later CSS weight requests
• report only the selected instance's actual weight from FontLibrary.family()

Why

Some variable fonts use multiple axes for a named face, such as optical size plus weight. Registering the variable file and later selecting only wght loses the named instance's other coordinates. This makes it impossible to register a face such as { opsz: 14, wght: 300 } faithfully.

The new API is additive:

FontLibrary.use("Example Light", {
  path: "/path/to/variable-font.ttf",
  namedInstance: 3,
})

Existing string-path registration is unchanged.

Validation

• npm test — 142 tests passed
• added coverage comparing a locked Raleway Light named instance with its static Light face across requested weights 300, 400, and 700
• manually verified a two-axis { opsz: 14, wght: 300 } named instance retains identical metrics when the caller requests other CSS weights