Add Bubble visual effect

[Copilot]

Adds a new Bubble effect: randomly-sized hollow bubble rings that rise from the bottom of the screen, with smooth fade in/out.

Effect properties

• color — bubble ring color (default: light blue, semi-transparent)
• count — number of bubbles in the pattern (5–200, default 20)
• speed — rise speed in screen heights/sec (default 0.15)
• max_size — maximum bubble radius as a fraction of screen height (default 0.06)
• length — resizable event duration in beats

Implementation notes

• Canvas/tiling approach: bubble positions (x, y, radius) are generated once into a fixed array using a local System.Random. Each frame the entire pattern is scrolled by Time.time * speed with modulo wrapping — no per-frame allocation or random generation.
• Seamless wrap: bubbles straddling the top or bottom edge are drawn at both their primary position and the adjacent tile edge to prevent popping.
• Ring shape: each bubble is an annulus (inner radius = 70% of outer) drawn as 16 × 2 GL triangles per bubble. Aspect ratio is corrected via rx = ry / cam.aspect so rings appear circular at any resolution.
• Fade envelope: 20% fade-in / hold / 20% fade-out, consistent with Fog and Vignette.

Files changed

• BopVisualEffects/Effects/Bubble/BubbleEffect.cs — new effect (definition + runner + overlay)
• BopVisualEffects/Core/EffectDefinitionRegistry.cs — register BubbleEffect alphabetically first
• docs/effects/README.md — document properties; preview video placeholder left for in-game recording

Original prompt

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This section details on the original issue you should resolve

<issue_title>Bubble effect</issue_title>
<issue_description>New effect, it adds bubble textures of randomized sized rising from the bottom, with possibility to configure how fast and how many bubbles there are + some scalę to how big they can get

Length of the event should be configurable.

To make sure that spawning a lot of bubbles doesn't cause performance issues, we can try an approach of rendering everything once to a Canvas and then scrolling it vertically with wrapping.

Should smoothly fade in at the start and then smoothly fade out at the end.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Bubble effect #28

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