Optimize radix route fallback matching

[heliang-whut]

Summary

This PR optimizes Radix tree route matching when a node has both static children and a wildcard fallback. The router now records the fallback node directly and skips the already-failed static branch when rolling back, avoiding the per-request node snapshot allocation. It also reuses the pre-trimmed path string instead of rebuilding it with prefix + path.

Benchmark report

Environment:

• OS/arch: darwin/arm64
• CPU: Apple M4 Max
• Command: go test -run=^$ -bench='Benchmark(StaticRouteWithParamFallback|OneRoute|ManyRoutes|404Many|Github)$' -benchmem -count=10
• Baseline: origin/master at 5f4f964, with the new benchmark added for comparison

Benchmark	Baseline ns/op	This PR ns/op	Change	Baseline allocs	This PR allocs
BenchmarkStaticRouteWithParamFallback	53.719	22.409	-58.28%	2 allocs / 128 B	0 allocs / 0 B
BenchmarkOneRoute	18.939	19.160	+1.17%	0 allocs / 0 B	0 allocs / 0 B
Benchmark404Many	34.153	34.589	+1.28%	0 allocs / 0 B	0 allocs / 0 B
BenchmarkGithub	580.950	585.870	+0.85%	15 allocs / 833 B	15 allocs / 833 B

The main optimized case removes all matching allocations and improves latency by about 58%. Existing broad router benchmarks remain allocation-neutral; timing changes are within about 1.3% in this local run.

Pull Request Checklist

• Open your pull request against the master branch.
• All tests pass in available continuous integration systems (local go test ./... passes; CI will run on GitHub).
• Tests are added or modified as needed to cover code changes.
• This PR does not introduce a new feature requiring docs/doc.md updates.

Verification

• go test -run='TestTree|TestRouter|TestRoute|TestGithub' -timeout=60s ./...
• go test ./...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.36%. Comparing base (3dc1cd6) to head (6d97ab9).
⚠️ Report is 276 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4654      +/-   ##
==========================================
+ Coverage   99.21%   99.36%   +0.14%     
==========================================
  Files          42       48       +6     
  Lines        3182     3137      -45     
==========================================
- Hits         3157     3117      -40     
+ Misses         17       13       -4     
+ Partials        8        7       -1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	99.35% <100.00%> (?)
-tags go_json	99.22% <100.00%> (?)
-tags nomsgpack	99.28% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	99.29% <100.00%> (?)
go-1.26	99.36% <100.00%> (?)
macos-latest	99.29% <100.00%> (+0.08%)	⬆️
ubuntu-latest	99.36% <100.00%> (+0.14%)	⬆️

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