Expose retained memory estimates

[mjackson]

• Add allocation-free retained-capacity estimates for url_aggregator and url_search_params
• Expose equivalent C APIs that include each opaque handle's owning result block
• Use saturating arithmetic so embedders can safely report external memory to their garbage collector

This lets embedders account for Ada-owned URL and query-list storage without inspecting private layout or allocating during GC.

[anonrig]

LGTM. Thank you for the PR

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.94%. Comparing base (761db09) to head (70730e9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/url_search_params.cpp	83.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1174      +/-   ##
==========================================
+ Coverage   59.79%   59.94%   +0.15%     
==========================================
  Files          37       38       +1     
  Lines        6121     6149      +28     
  Branches     2978     2982       +4     
==========================================
+ Hits         3660     3686      +26     
  Misses        618      618              
- Partials     1843     1845       +2     

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[lemire]

@mjackson What is the purpose here?

Will every little component of out software systems have a memory estimator bundled with it?

What function does the estimator serve?

How often are systems memory limited because they are storing too much data related to URLs?

Are you going to be using this personally in a significant manner? Can you describe the usage you have in mind?