ref(snapshotting): Centralize preview filename resolution

[cameroncooke]

Centralize snapshot export filename resolution behind a single resolver and simplify the filename policy used by SnapshotTest.

The previous logic was split across discovery-time metadata maps, runtime assembly in testPreview, and several fallback branches. That made the code harder to reason about and also produced unstable filenames in cases where a display name was already enough to identify the preview.

This change moves filename generation into SnapshotTest.FileNameResolver, computes names once during preview discovery, and makes the runtime path a simple lookup.

Behavior changes:

• Named #Previews now prefer fileId + displayName
• Duplicate named previews now use displayName_ordinal
• Anonymous macro previews fall back to line-<line>
• Anonymous provider previews still fall back to previewIndex
• Duplicate line information is no longer emitted in filenames

Before / after examples:

• Before: HackerNews_CommentRow.swift_152_HTML_case_1.png

• After: HackerNews_CommentRow.swift_HTML_case_1.png

• Before: Feature_LoginView.swift_42_line-42.png

• After: Feature_LoginView.swift_line-42.png

• Before: duplicate named provider previews could fall back to raw indexes such as Test_View_3

• After: they resolve to display-name ordinals such as Test_View_Dark_Mode_1

I also renamed the internal count from duplicateDisplayNameCount to displayNameOccurrenceCount and added typealiases for the resolver's string-backed grouping concepts so the code reads closer to what it is actually modeling.

Validated with swift test --filter SnapshottingTestsTests and swift test.