Add IAsyncAlternateLookup support to ConcurrentLfu

[Copilot]

Adds IAsyncAlternateLookup support to ConcurrentLfu and ConcurrentTLfu, mirroring the existing implementation on ConcurrentLru.

Core changes

• ConcurrentLfuCore.cs: AlternateLookup<TAlternateKey> now implements both IAlternateLookup and IAsyncAlternateLookup in a single struct. Adds GetOrAddAsync, GetOrAddAsync<TArg>, and a private GetOrAddAsyncSlow helper. Since TAlternateKey allows ref struct, the public methods are non-async wrappers that capture the ref struct before delegating to the async slow path via a plain K key.
• ConcurrentLfu.cs: Exposes GetAsyncAlternateLookup<TAlternateKey>() and TryGetAsyncAlternateLookup<TAlternateKey>().
• ConcurrentTLfu.cs: Exposes GetAsyncAlternateLookup<TAlternateKey>() and TryGetAsyncAlternateLookup<TAlternateKey>(), delegating to the inner core, matching ConcurrentLfu.

Tests

• ConcurrentLfuAsyncAlternateLookupTests.cs (new): unit tests for comparer compatibility, and GetOrAddAsync/GetOrAddAsync<TArg> miss+hit behaviour.
• ConcurrentTLfuTests.cs: unit tests for GetAsyncAlternateLookup and TryGetAsyncAlternateLookup covering compatible and incompatible comparer cases.
• ConcurrentLfuSoakTests.cs: two new concurrent soak tests for GetOrAddAsync and GetOrAddAsync<TArg> using ReadOnlySpan<char> keys.

Usage

var cache = new ConcurrentLfu<string, Data>(1, 1000, scheduler, StringComparer.Ordinal);
var lookup = cache.GetAsyncAlternateLookup<ReadOnlySpan<char>>();

ReadOnlySpan<char> key = "some-key".AsSpan();
var value = await lookup.GetOrAddAsync(key, static k => FetchAsync(k.ToString()));

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[coveralls]

coverage: 99.158% (+0.04%) from 99.118%
when pulling d02e3a8 on copilot/implement-iasyncalternatelookup
into 0ec5a06 on main.

[bitfaster]

@copilot Add the GetAsyncAlternateLookup and TryGetAsyncAlternateLookup methods to ConcurrentTLfu, matching ConcurrentLfu. Add simple unit tests for ConcurrentTLfu to verify the correct object is returned and they are wired up to the inner class.

[Copilot]

@copilot Add the GetAsyncAlternateLookup and TryGetAsyncAlternateLookup methods to ConcurrentTLfu, matching ConcurrentLfu. Add simple unit tests for ConcurrentTLfu to verify the correct obje...

Done in d02e3a8. Added GetAsyncAlternateLookup and TryGetAsyncAlternateLookup to ConcurrentTLfu, both delegating to core, matching the pattern used in ConcurrentLfu. Added 4 unit tests to ConcurrentTLfuTests.cs covering compatible/incompatible comparer cases for both methods.