perf(shell): bound the billable collection concurrency and cover it

[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	26 of 27
Base	upstream/statistics-25-billable-goldens-and-optin
Depends on	#53
Follow-up	upstream/statistics-26-billable-bounded-collection (slice 27)
Review budget	60 / 400
Starts at	upstream/statistics-25-billable-goldens-and-optin
Ends with	perf(shell): bound the billable collection concurrency and cover it

Chain Overview

upstream/main
 └── upstream/statistics-tracker (root, integration PR #1627)
      └── … slices 1..25 … (##53 is the immediate parent)
           └── 📍 upstream/statistics-26-billable-bounded-collection
                └── … slices 27..27 …

Scope

• Includes: perf(shell): bound the billable collection concurrency and cover it
• Excludes: every other slice. A polluted diff against upstream/statistics-25-billable-goldens-and-optin 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.