docs(router): align RouteMatch API reference with the current public contract

[LadyBluenotes]

Goal

Bring the RouteMatch API reference in line with the current public contract.

Scope

• Remove obsolete showPending and fetchedAt entries from docs/router/api/router/RouteMatchType.md.
• Add fullPath using its route-specific type, Route['fullPath'].
• Add index and explain its ordering within the matched route array.
• Update cause to include 'preload'.
• Keep internal and lifecycle-specific match state out of the reference, including displayPendingPromise, fetchCount, preload, invalid, headers, and globalNotFound.
• Review the complete page against packages/router-core/src/Matches.ts so the reference describes the intentional public API.

Related Work

This continues the RouteMatch reference update started in #4894

[soheilnikroo]

@LadyBluenotes I’ve audited RouteMatchType.md against packages/router-core/src/Matches.ts and the feedback from #4894.

Changes:

Removed obsolete showPending and fetchedAt
Added fullPath typed as Route['fullPath'] (not plain string)
Added index with docs explaining root-to-leaf ordering in the matches array
Updated cause to include 'preload'
Excluded internal/lifecycle-only fields (displayPendingPromise, fetchCount, preload, invalid, headers, globalNotFound, and _-prefixed properties)
Documented other intentional public fields used by hooks/guides (loaderDeps, staticData)