refactor: No normalized path interning

[nielsenko]

Description

Remove NormalizedPath interning

This PR removes NormalizedPath interning completely.

Interning was originally added to avoid re-normalizing paths, but it always had two problems:

• A rather bad perf cliff: some inputs thrash the cache (Performance cliff on high-cardinality dynamic paths due to NormalizedPath interning #342).
• Per-isolate memory use: Dart makes it impossible (or very difficult) to share caches across isolates (Investigate cheaper no-interning and multi-isolate cache sharing for NormalizedPath #344).

On top of that, the new NormalizedPath.fromSegments and NormalizedPath.fromUri constructors - now the ones actually used on the request path - made interning tricky, so the security PRs (#369 & #371) simply skipped it for those paths.

Most of the lost performance is recouped by:

• Pre-computing repeated work.
• Extracting parameters into a single mutable map with undo-in-place during backtracking. Previously a copy was made at each parameter level so backtracking could just throw it away, but undoing in place avoids the copy and the allocation, reducing GC pressure.
• Inferring from the registered routes whether backtracking is needed at all, and taking a faster non-backtracking walk when it isn't.

Results

┌────────┬─────────┬─────────┬────────────┐
│ routes │ Router  │ Spanner │ Routingkit │
├────────┼─────────┼─────────┼────────────┤
│ 1M     │ 1229 ns │ 1393 ns │ 3468 ns    │
├────────┼─────────┼─────────┼────────────┤
│ 100k   │ 994 ns  │ 1015 ns │ 3194 ns    │
├────────┼─────────┼─────────┼────────────┤
│ 10k    │ 579 ns  │ 402 ns  │ 1889 ns    │
└────────┴─────────┴─────────┴────────────┘
(measured on a 2.4 GHz 8-core Intel Core i9)

Fixes: #118 (not by improving caching, but by dropping it)
Fixes: #342 (this time by disabling interning altogether, instead of making caching opt-out)
Closes: #344 (investigation done)

Breaking changes

• NormalizedPath.interned is removed

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.72%. Comparing base (a623f2d) to head (ca653c6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #375      +/-   ##
==========================================
- Coverage   92.74%   92.72%   -0.02%     
==========================================
  Files         110      110              
  Lines        4687     4702      +15     
  Branches     2373     2380       +7     
==========================================
+ Hits         4347     4360      +13     
- Misses        340      342       +2     

Flag	Coverage Δ
relic_core	92.68% <100.00%> (-0.03%)	⬇️
relic_io	93.11% <ø> (ø)

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[FXschwartz]

LGTM! 🚀