perf: enumerate result pages without boxing

[msallin]

SvgResult and PngResult implement IReadOnlyList<T> by forwarding GetEnumerator() to the list behind their Pages property. Because Pages is typed as the interface, the list hands its enumerator back as an IEnumerator<T>, so the struct is boxed on every foreach. Measured: 80 bytes for one pass over an SvgResult and a PngResult, 40 each.

A struct enumerator, which foreach binds to in preference to the interface, takes that to zero. The interface implementations remain, now explicit, so LINQ and IEnumerable callers are unaffected.

Adds one public type, PageEnumerator<T>, shared by both results rather than duplicated as two nested structs.

Compatibility

Source-compatible: foreach, LINQ, Count and the indexer all keep compiling, and result.GetEnumerator() still converts to IEnumerator<T> where a caller assigned it to one.

Binary-breaking: the public IEnumerator<T> GetEnumerator() becomes an explicit interface implementation. This needs a version bump rather than a patch release, so it should be scheduled rather than merged alongside the other allocation fixes.

Tests

• EnumeratingResultPagesDoesNotAllocate asserts zero allocations across one pass over both result types. Confirmed failing at exactly 80 bytes against the previous implementation.
• ResultPagesEnumerateInIndexOrderThroughBothPaths pins that the struct enumerator agrees with the indexer, and that the interface path LINQ uses still yields the same pages.

58/58 tests pass. Builds clean on net8, net9 and net10 with zero warnings.

[msallin]

Reopened upstream against the parent repository: evolvedlight#50