style: satisfy the swiftlint strict gate in the operation reporting code

[datlechin]

swiftlint lint --strict fails on main right now, on three violations that arrived with #2272:

TablePro/Core/Services/Operations/OperationDurationFormatter.swift:16:36: error: Number Separator Violation
TablePro/Core/Services/Operations/OperationDurationFormatter.swift:17:39: error: Number Separator Violation
TablePro/Core/Coordinators/RowEditingCoordinator+SaveChanges.swift:394:1: error: Extension Access Modifier Violation

That gate is the lint job in build.yml, and release needs it, so the next v* tag would have failed on this rather than on anything the tag introduced.

It merged unseen because PR CI never runs SwiftLint. macos-tests.yml builds and tests; only build.yml lints, and only on a tag. Worth closing separately, but this PR just clears the red.

The fix

• 3600 becomes 3_600 in both places.
• fileprivate moves from reportSaveFinished onto the extension that holds it, which is what extension_access_modifier asks for and what CLAUDE.md specifies under Code Style. The extension has one member, so the effective access is unchanged.

No behaviour change, so no CHANGELOG entry.

Verification

• build PASS
• lint TablePro PASS, 0 violations (was 3 errors)
• test PASS, 111 of 111 across the suites touching the coordinators involved

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