Completes the Effects-controls gap — each effect row now exposes its
editable scalar parameters:
- op-editor-core: `EffectField` enum + `EditorCommand::SetEffectParam`
+ `cmd_set_effect_param` — writes one shadow param (offset X/Y,
blur, spread) or a blur/background-blur radius; blur values clamp
to >= 0, field/effect mismatches reject.
- panels: each effect block paints a parameter row per field —
`<label> <value> [-] [+]`; the "-"/"+" steppers emit
`AdjustEffectParam` (the walker computes the post-step value from
the current one). `effects_section_height` / the action-rect
walker now take the effects slice so the variable per-kind block
height stays aligned with paint (`VisibleSections.effect_count`
retired in favour of the slice).
- both hosts dispatch `AdjustEffectParam` via `SetEffectParam`,
history-committed.
op-editor-core 233 / op-editor-ui 140 / op-host-desktop 64 tests
green.
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