[http_server] Cache parameter child pointer for O(1) lookup fallback

[cong1920]

Summary

Cache the {{param}} child pointer during route registration (ind_or_create) so that ind() can fall back to it in O(1) instead of doing a linear scan of all children.

Problem

In ind(), when a static child lookup misses, the code did a linear scan of ALL children to find a {{param}} child:

\\cpp
for (const auto& kv : children_) {
if (name.size() > 4 and name[0] == '{' and name[1] == '{' ...)
return kv.second->find(r, c);
}
\\

This is O(n) per tree level for every parameterized route lookup.

Fix

Added a cached pointer set during \ind_or_create():

\\cpp
drt_node* param_child_ = nullptr;
\\

Now \ind()\ falls back in O(1):

\\cpp
if (param_child_)
return param_child_->find(r, c);
return end();
\\

Benchmark Results

Environment: WSL2 Ubuntu, g++ -O2 -std=c++20, 25 iterations

Metric	Before	After	Change
insert	313.3 ns/op	299.2 ns/op	-4.5%
lookup hit	80.7 ns/op	72.7 ns/op	-9.9%
lookup miss	45.0 ns/op	36.6 ns/op	-18.7%

vs master branch

Metric	master	This PR	Change
insert	268.2 ns/op	299.2 ns/op	+11.6% (structural cost of simplify branch)
lookup hit	80.7 ns/op	72.7 ns/op	-9.9% faster
lookup miss	45.9 ns/op	36.6 ns/op	-20.3% faster

The hot path (lookups) now beats master by 10-20%.

[cong1920]

Committed directly to simplify_drt_node instead.