fix: Mark the reversed() cache as valid after rebuilding it

[spydon]

Description

_validReverseCache starts out true and is set to false on every mutation (add, remove, clear), but it was never set back to true when reversed() rebuilt the cache. As a result, after the first mutation every single reversed() call did a full toList(growable: false) of the set, and the "cache" never served a single hit.

This is a significant hidden cost for Flame, where Component.componentsAtLocation calls children.reversed() for every component with children on every pointer event, so every game pays an O(n) list materialization per node per hit test.

The fix marks the cache valid after rebuilding it. Invalidation behavior is unchanged: any mutation still resets the flag, and the existing reversed properly invalidates cache tests still pass. Added a regression test to both implementations asserting that the returned iterable is identical between calls when no mutation happened, and is correctly rebuilt after a mutation.

Found while working on flame-engine/flame#3957.

[github-actions]

Benchmark Results

Package ordered_set:

Benchmarks	Current Branch [fix/reversed-cache-never-valid]	Base Branch [main]	Diff
Iteration Benchmark - Comparing	240.157 μs	249.320 μs	🟢 -3.675 %
Insertion and Removal Benchmark - Comparing	520.705 μs	522.828 μs	🟢 -0.406 %
Comprehensive Benchmark - Comparing	16226.411 μs	16327.258 μs	🟢 -0.618 %
Iteration Benchmark - Priority	258.616 μs	254.085 μs	🔴 +1.783 %
Insertion and Removal Benchmark - Priority	340.519 μs	341.264 μs	🟢 -0.218 %
Comprehensive Benchmark - Priority	15486.938 μs	15441.366 μs	🔴 +0.295 %

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Benchmarks provided with 💙 by Dart Benchmark Action.

[coveralls]

Coverage Report for CI Build 29873276610

Coverage increased (+0.03%) to 97.537%

Details

• Coverage increased (+0.03%) from the base build.
• Patch coverage: 2 of 2 lines across 2 files are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	203
Covered Lines:	198
Line Coverage:	97.54%
Coverage Strength:	3.07 hits per line

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💛 - Coveralls

[luanpotter]

LGTM, shocked it was never set!