Add Menu::collapseAfter() for overflow dropdown

[WarLikeLaux]

Q	A
Is bugfix?	❌
New feature?	✔️
Docs added?	❌
Tests added?	✔️
Breaks BC?	❌

What does this PR do?

Adds Menu::collapseAfter() to keep the first N visible items in place and move the rest into a "More" dropdown.

Menu::widget()
    ->collapseAfter(4)
    ->collapseLabel('More')
    ->items([...])
    ->render();

New methods: collapseAfter(?int), collapseLabel(string), and collapseDropdownDefinitions(?array). The default collapseAfter value is null, so all items stay visible. Invisible items don't count toward the threshold. The collapse dropdown reuses dropdownDefinitions() unless collapseDropdownDefinitions() is set separately.

No BC break: the new behavior is disabled by default, and existing menu output is unchanged unless collapseAfter() is set.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (c3ae973) to head (93903d0).

Additional details and impacted files

@@             Coverage Diff             @@
##              master      #141   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity       327       342   +15     
===========================================
  Files              8         8           
  Lines           1038      1082   +44     
===========================================
+ Hits            1038      1082   +44     

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[samdark]

Code review findings:

• Bug: Collapsed item labels are normalized twice, which breaks escaping and icons. In src/Menu.php, collapsed raw items are first normalized as Menu items, then renderDropdown() sends them through Dropdown::render(), which normalizes them again. Example: Black & White renders as Black & White, encode => false is ignored, and icon markup renders as escaped text. Please add collapse tests for encoded labels, encode => false, and icons.

• Contract mismatch: Menu::collapseAfter() is documented/stored as positive-int|null, but it accepts 0 and negative values without validation. Both currently collapse every item into “More”, which conflicts with the declared contract and with the existing Breadcrumbs::maxItems() validation pattern.

Checks run locally on the PR branch:

• vendor/bin/phpunit tests/Menu/MenuTest.php --testdox passed.
• vendor/bin/psalm --no-cache --output-format=console passed.
• Targeted renders confirmed the double-encoding/icon regression and collapseAfter(0/-1) behavior.