refactor(pipeline): split effects.rs into focused sub-modules

[srperens]

Summary

effects.rs grew past the 1500-line guideline (1,756 lines) with the crop/zoom work in #622. This splits it into focused sub-modules following the builder/ pattern — a pure move with only import/visibility adjustments, no logic changes:

Module	Responsibility	Lines
effects/take.rs	The PGM take engine (trigger_transition, PiP-aware + classic paths)	~420
effects/mixer_ops.rs	Vision mixer runtime ops (bus selection, DSK, FTB, overlay alpha, PiP config, input resolutions)	~930
effects/mixer_layout.rs	Pad geometry/crop appliers shared by the take engine, mixer ops and the reactive geometry probe	~280
effects/misc.rs	Block-generic effects (animate input, loudness reset, recorder split, thumbnail capture)	~150
effects/mod.rs	Shared dist_canvas_size + crate-facing re-exports	~45

Crate-facing paths are unchanged (crate::gst::pipeline::effects::{find_pad, apply_*} re-exported from mod.rs), so geometry.rs and other consumers compile untouched.

Test plan

• cargo check clean, zero unused-import warnings
• vision mixer unit tests (28 + 32) green
• pipeline_lifecycle_test (leak regression) green

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