refactor: speed up option selection state in useSelect

[TrevorBurnham]

This PR speeds up the selection-state computation that runs on every render in useSelect, which backs both Select and Multiselect.

Previously, useSelect recomputed which options are selected using two nested scans that grew quadratically with the size of the options and selection lists:

• connectOptionsByValue scanned all dropdown options to find a match for each selected option — O(selected × options).
• getOptionProps then ran up to three __selectedOptions.indexOf() scans per rendered option (the option itself plus its two neighbours, used to compute selected / isNextSelected / isPreviousSelected) — O(filtered × selected). On the default, non-virtualized PlainList this scans the entire list, so the cost is not bounded by the visible window.

For a large multiselect, this adds up to hundreds of microseconds of avoidable work on every render.

This change makes both lookups linear:

• connectOptionsByValue builds a Map of dropdown options keyed by value once, then resolves each selected option in O(1) → O(selected + options).
• useSelect builds a Set of the selected dropdown options once per render and tests membership in O(1) per option, replacing the per-option indexOf scans.

The Set is keyed by the same DropdownOption references that connectOptionsByValue returns, so Set.has() is exactly equivalent to the previous reference-based indexOf() > -1 checks — including the undefined-neighbour case at list boundaries.

Benchmarks (Node, isolated microbenchmarks with an equivalence check):

Function	Inputs	Before	After	Speedup
connectOptionsByValue	1000 options, 50 selected	147µs	13µs	~11x
connectOptionsByValue	2000 options, 200 selected	993µs	27µs	~37x
getOptionProps (over all options)	1000 options, 100 selected	50µs	12µs	~4x

How has this been tested?

• New unit tests in src/select/__tests__/connect-options.test.ts cover the edge cases that must be preserved: empty selection, order preservation, parent options skipped, first-match-wins on duplicate values, and matching options with an undefined value.
• The selection-state change in getOptionProps is covered by the existing src/select/__tests__/use-select.test.ts assertions on selected / isNextSelected / isPreviousSelected, including the group-neighbour case.
• Full Select, Multiselect, and Autoselect unit suites pass (src/select/, src/multiselect/, src/autosuggest/ — 552 tests green).

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[gethinwebster]

Looks good but please remove the inline comments, they're adding unnecessary explanations.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.50%. Comparing base (55c9250) to head (e433965).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4611   +/-   ##
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  Coverage   97.50%   97.50%           
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  Files         948      948           
  Lines       30314    30317    +3     
  Branches    11046    11047    +1     
=======================================
+ Hits        29559    29562    +3     
+ Misses        748      708   -40     
- Partials        7       47   +40     

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