feat: add routeKey to getRoute return type

[olliethedev]

Note

Medium Risk
Adds a new public field and runtime lookup to route matching; while behavior is additive, the handler-reference-based key resolution could misbehave if route handlers are reused or wrapped.

Overview
createRouter().getRoute now includes a routeKey identifying which entry in the routes map matched, and its TypeScript return type is updated to preserve narrowing based on that key.

Adds a test suite covering routeKey for static, parameterized, and overlapping routes, and bumps the package version to 1.2.0.

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