fix(crossrepo): treat only "::" as a namespace marker

[dejo1307]

linkSharedSymbols guards against linking two repos in different languages that merely share a domain type name. The guard is skipped for identities it considers namespace-qualified, and isQualifiedIdentity counted any dot as qualifying.

By the time it runs, typeIdentity has already stripped the module prefix, so a residual dot means type nesting, not a namespace. Kotlin and Swift both emit nested declarations as "Outer.Inner". Two apps that model the same product in different languages therefore cleared minSharedSymbols on nested names alone, bypassed the guard, and were given a bidirectional depends_on -- a 2-cycle in the service graph, and an edge no parser justifies. traverse, find_path and impact_analysis route through it, and coverage_report overstates each app's outbound edge count.

Replace isQualifiedIdentity with isNamespaceQualified, matching "::" only, at both call sites: the same-language guard and the isDistinctiveIdentity short-circuit. Nested identities are now unqualified, so they fall under the language guard and under the length/generic-name filters.

Coverage:

• TestComputeLinks_SharedSymbolsCrossLanguageNestedTypesSkipped, which failed before this change with the exact bidirectional pair.
• TestComputeLinks_SharedSymbolsSameLanguageNestedTypesLink, guarding the over-correction: a nested identity is now unqualified and so newly depends on the language guard; same-language repos must still link.
• testdata/repos/kotlin_swift_multirepo, the first golden fixture pairing two different-language repos. The bug lives between what the extractors name symbols and what the linker assumes a dot means, and every input in crossrepo_test.go is a hand-built fact, so no synthetic-fact test can reach it. Verified by reinstating the bug and watching the golden fail.

No cacheVersion bump: linkCrossRepo runs after runExtractors and after the extractor cache is saved, so no extractor output changes. The linker's synthetic facts do land in facts.jsonl, so goldens were regenerated; the pre-existing 16 are byte-identical.