Clarify Team presentation ownership

[Mattsface]

Summary

Continues GitHub issue #30 with a small presentation-boundary cleanup after PR #54.

• renames the Team-specific selector partial from _selector_form.html to _team_selector_form.html
• makes Team/season/window ownership explicit without generalizing the selector for future Player UI
• extracts the genuinely repeated summary-card markup into a small _summary_cards.html partial
• preserves Comparison's distinct summary class and accessible label
• documents the Team-specific selector boundary
• updates focused tests for selector structure, empty-state behavior, and summary-card rendering
• fixes the stale selector-partial reference in the strikeouts architecture doc

Preserved behavior

This PR does not change:

• Team routes or canonical URLs
• team_id, season, or window query behavior
• selector behavior or season-selector.js
• navigation introduced by PR Organize Team analytics navigation #54
• chart rendering or analytics
• MLB-average completeness semantics
• empty, 404, 409, or comparison-unavailable behavior
• persistence or ingestion
• Player UI
• the DB-only browser guarantee

Scope

The application shell remains entity-neutral. Team pages explicitly own Team/season/window selection, while the summary partial only renders the existing route-provided summary_cards data.

No generic frontend/component framework is introduced.

Part of #30.