Add TextPool: keyed cache of reusable Text objects

[pvcraven]

Summary

• Adds TextPool class to arcade/text.py — a keyed cache of reusable Text objects that avoids the cost of creating new arcade.Text objects every frame for dynamic text (scores, labels, FPS counters, etc.)
• draw(key, text, x, y, ...) creates on first call, efficiently updates on subsequent calls, then draws
• get(key, text, x, y, ...) same but without drawing — useful for measuring content_width before layout
• Pool-wide defaults (font_name, bold, anchor_x, etc.) set in constructor, overridable per-call
• Uses Text.__enter__/__exit__ context manager for efficient batched property updates via pyglet's begin_update()/end_update()

Test plan

• 12 unit tests covering: creation, reuse (same id()), property updates, get() vs draw(), clear(), remove(), defaults, overrides, content_width access
• All 36 existing text tests still pass
• from arcade import TextPool works

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