fix(group): preserve explicit 404 routes across Use calls

[tianrking]

Description

Calling Group.Use after registering a custom catch-all RouteNotFound currently replaces that handler with the default 404. Preserve the successfully registered group route and its route-level middleware when rebuilding the catch-all with the latest group middleware. This also retains the route name, copies the middleware slice, and discards saved routes that have been removed.

The regressions cover both catch-all paths, registration order, repeated Use, rejected registrations, middleware order and ownership, disabled automatic registration, route removal, and an actual HTTP request with connection cleanup.

Closes #3153.

Validation

• Native original-production replay: 8 failing scenarios and 5 passing controls; the corrected code passes all 13 scenarios.
• Exact source commit f6051c1c9c844d35c88776fa45a3e657480f7c24: full race and coverage tests on Go 1.25.14 and 1.27.1, covering all three packages, with no failed or skipped tests.
• make check, explicit golint -set_exit_status ./..., staticcheck ./..., go vet ./..., go test -short -count=1 ./..., govulncheck ./..., and changed-file gofmt checks pass.
• Complete original/fixed benchmarks ran with eight samples at 100ms each; the comparison is advisory, as in the project's CI.

Hosted native validation and source hashes, benchmark comparison using the unchanged recorded measurements.

[vishr]

Thanks for the PR. Closing along with #3153. The order dependence is by design (#2411), so we're documenting it rather than adding per-group tracking state.