Summary

Fixes critical routing bug where routes with :param followed by multiple static segments would panic.

Problem

Routes like /users/:id/activate + /users/:id/deactivate caused panic at line 147 in tree.go:

panic: internal error: no common prefix between "/deactivate" and ":id"

Root Cause

Common prefix check compared /deactivate with :id and found no match.

Solution

Implemented httprouter's production-quality pattern:

• Create empty placeholder child nodes after param nodes
• Bypass common prefix check for param node children
• Use direct children assignment (n.children = []*node{child}) to allow empty path placeholders

Changes

• internal/radix/tree.go (lines 118-138): Added special case handling for param node children
• internal/radix/tree_param_static_test.go (new file): 6 comprehensive test cases
• CHANGELOG.md: Documented v0.3.3 release

Testing

✅ All 650+ tests passing with race detector (WSL2 Gentoo)
✅ Coverage: 88.4% internal/radix, 91.7% overall (maintained)
✅ Linter: 0 issues (golangci-lint clean)
✅ Performance: No regressions (256 ns/op static, 326 ns/op parametric)
✅ CI: GREEN

Test Cases Added

1. TestTree_ParamWithMultipleStaticChildren - Original bug case
2. TestTree_NestedParams - Nested params (/users/:user_id/posts/:post_id/comments)
3. TestTree_AlternatingParamStatic - Alternating pattern (/a/:b/c/:d/e)
4. TestTree_ParamWithLongStaticTail - Long static tails (/:a/b/c/d/e/f/g)
5. TestTree_ConsecutiveParams - Consecutive params (/:a/:b/:c/d)
6. TestTree_StaticVsParam - Static priority over params

Reference

Studied httprouter implementation (tree.go lines 217-270, 180-184) for production-quality reference solution.

Checklist

• Tests added and passing
• Race detector passed
• Linter clean
• CHANGELOG updated
• Performance validated
• CI passing