Fix Fetch dropping repeated form fields and leaking src into history (2.x)

[titouanmathis]

Cherry-pick of 7edbbdd from #643, which landed on main. Fetch.ts was byte-identical on the two branches, so it applied without conflict.

See #643 for the reasoning. In short:

• a repeated GET form field sent one value, because the fold used set per entry — a checkbox group could never send more than one term;
• history pushed the URL that was fetched, so a src pointing at a lighter endpoint leaked its parameters into the address bar. The new historyUrl getter follows the element's own destination instead.

Nothing changes for an element without src.

Verified on this branch: 100 Fetch tests, full suite green.

[github-actions]

Code Review

Risk: Medium — issues that should be addressed before merge.

This change preserves repeated GET form values and separates the requested URL from the URL written to browser history. It adds URL-resolution coverage and documentation for both behaviors, but the new history state persists across updates and can override URLs supplied to later direct update() calls.

1 issue found:

• issue — packages/ui/src/Fetch/Fetch.ts:501 — Clear stale history URL before direct updates

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Review usage: 19,961 in (3,677 cached) / 696 out tokens — $0.0124 (openrouter/openai/gpt-5.6-luna, thinking: low)

Reviewed by @weareikko/code-review v0.9.5 for commit a8e95f7.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.85%. Comparing base (c939977) to head (a8e95f7).

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x     #644      +/-   ##
============================================
+ Coverage     87.80%   87.85%   +0.05%     
  Complexity      145      145              
============================================
  Files           153      153              
  Lines          5443     5460      +17     
  Branches       1037     1040       +3     
============================================
+ Hits           4779     4797      +18     
+ Misses          580      579       -1     
  Partials         84       84              

Flag	Coverage Δ
unittests	87.85% <100.00%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
packages/ui/src/Fetch/Fetch.ts	100.00% <100.00%> (+0.70%)	⬆️

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