test(statistics): cover the exporters and add the export golden files

[efirvida]

Chain Context

Field	Value
Chain	per-session statistics module (upstream Gentleman-Programming#1583)
Tracker	upstream/statistics-tracker → upstream integration PR Gentleman-Programming#1627 (draft)
Position	17 of 27
Base	upstream/statistics-16-exporters-and-contract
Depends on	#44
Follow-up	upstream/statistics-17-exporter-tests (slice 18)
Review budget	335 / 400
Starts at	upstream/statistics-16-exporters-and-contract
Ends with	test(statistics): cover the exporters and add the export golden files

Chain Overview

upstream/main
 └── upstream/statistics-tracker (root, integration PR #1627)
      └── … slices 1..16 … (##44 is the immediate parent)
           └── 📍 upstream/statistics-17-exporter-tests
                └── … slices 18..27 …

Scope

• Includes: test(statistics): cover the exporters and add the export golden files
• Excludes: every other slice. A polluted diff against upstream/statistics-16-exporters-and-contract is a branching bug, not a scope surprise.

Verification

Focused and affected suites are green on this slice; tests and docs for this unit ship with it.