feat(statistics): local opt-in JSONL store with bounded retention

[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	12 of 27
Base	upstream/statistics-11-timeline-tests
Depends on	#39
Follow-up	upstream/statistics-12-statistics-store (slice 13)
Review budget	290 / 400
Starts at	upstream/statistics-11-timeline-tests
Ends with	feat(statistics): local opt-in JSONL store with bounded retention

Chain Overview

upstream/main
 └── upstream/statistics-tracker (root, integration PR #1627)
      └── … slices 1..11 … (##39 is the immediate parent)
           └── 📍 upstream/statistics-12-statistics-store
                └── … slices 13..27 …

Scope

• Includes: feat(statistics): local opt-in JSONL store with bounded retention
• Excludes: every other slice. A polluted diff against upstream/statistics-11-timeline-tests 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.