feat: add KeysSeq and ItemsSeq range-over-func iterators

[ChrisJr404]

This adds the iterator functions from the v3.5 idea list in #212 ("Add iterator funcs for Keys() etc"), following the strings.FieldsSeq naming convention referenced there.

What

Two range-over-func iterators, the lazy counterparts of the existing Keys and Items methods:

• KeysSeq() iter.Seq[K] — yields the key of each unexpired item.
• ItemsSeq() iter.Seq2[K, *Item[K, V]] — yields the key and item of each unexpired item.

for key := range cache.KeysSeq() {
    // ...
}

for key, item := range cache.ItemsSeq() {
    // ...
}

Notes

• Both are implemented on top of Range, so they inherit its exact behaviour: unexpired items only, most-to-least-recently-added order, the cache lock released while a value is yielded (so calling other cache methods from within the loop is safe), and early termination when the loop breaks.
• Unlike Keys/Items, they do not allocate an intermediate slice or map.
• I kept the Seq suffix (rather than shadowing Keys/Items) both because those names are taken and to match the strings.FieldsSeq example from the issue.
• The module is already on go 1.25, so iter is available without any version bump.

Tests cover full iteration, expired-item exclusion, ordering, early break, empty caches, and calling cache methods during iteration (all with -race). README updated with a short example. go build, go vet, and go test -race ./... all pass.

Happy to adjust naming or split ItemsSeq to yield values instead of items if you prefer a different shape.

Refs #212

[coveralls]

Coverage Report for CI Build 32181612747

Coverage increased (+0.004%) to 99.745%

Details

• Coverage increased (+0.004%) from the base build.
• Patch coverage: 10 of 10 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

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Coverage Stats

Relevant Lines:	783
Covered Lines:	781
Line Coverage:	99.74%
Coverage Strength:	26.52 hits per line

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