var MatchedRoutePathParam = "$matchedRoutePath" changed declaration to const MatchedRoutePathParam

[LeoDiKadyrov]

I only suggested it due to following reasons:

• John Carmack once said: "I am a full const nazi nowadays, and I chide any programmer that doesn’t const every variable and parameter that can be."
• I'm a newbie into development, trying to learn staff and while studying source code of this library, I saw an opportunity for small change and my first PR into open source ever

All tests that are originally in master repo passed green. When I was going through the code it just confused me a bit, that a variable only used in router.go and not really mutated anywhere declared as var. Sorry if it's too small to have anyone attention and a whole PR open for it.

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UPD 23.03.2026
Breaking Change: Switching MatchedRoutePathParam from var to const. Any downstream code that previously reassigned httprouter.MatchedRoutePathParam will no longer compile.

[themavik]

Switching MatchedRoutePathParam from var to const is the right default for an immutable sentinel. nit: any code that reassigned httprouter.MatchedRoutePathParam will no longer compile—call that out in the PR body for downstream grep.

[LeoDiKadyrov]

Oh wow, unexpected monday surprise. Appreciate that you directed your attention to my PR. Will call it out in the PR body