test(path): cover Optional and Or iterator behavior (#238)

[JSap0914]

Closes #238

What

Adds narrowly scoped integration coverage for the Optional and Or iterators, addressing the low iterator coverage noted in #238. The new cases live in the shared path test set (query/path/pathtest), which is executed against every quadstore backend, so the coverage applies broadly rather than to a single store.

Why

#238 observes that the base iterators are under-covered by the integration tests, specifically the Optional and Or iterators. The most maintainable place to exercise these end-to-end today is the shared testSet in query/path/pathtest, driven via Save/SaveOptional/Optional/Or on the query path (the modern equivalent of the old top-level integration queries the issue refers to).

Coverage added

Against the standard data/testdata.nq graph:

• Or union of overlapping branches — alice->follows and dani->follows both reach bob; documents that Or is a union without deduplication (bob appears from each branch).
• Or preserves the Save tag of the producing branch — a value keeps the tag set by whichever Or branch produced it.
• SaveOptional keeps unmatched starts with an empty tag — starts without a matching quad survive with an empty tag while matched ones carry the value.
• Optional branch that never matches — every top-level node is still returned when the optional sub-path matches nothing.

Each case runs in both optimized and unoptimized iterator modes (the existing harness runs both), so the assertions pin behavior across optimization.

Verification

gofmt -l query/path/pathtest/pathtest.go   # clean
go vet ./query/path/                        # clean
go test ./query/path/                       # ok
go test ./graph/memstore/                   # ok (runs the shared path test set)

No production code changes; test-only plus a CONTRIBUTORS entry per the project's CONTRIBUTING guidance.

Refs #238

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