Plan: Brush Editor — Dynamics, Real Preview, Scrollable Panels

Goal

Extend the existing Krita-style brush editor in hcie-egui-app/crates/hcie-gui-egui/src/app/brush_editor.rs so it exposes real brush dynamics, shows an accurate engine-rendered preview, and keeps all dock panels scrollable by mouse wheel / pan without visible scrollbars.

Current State

A basic brush editor modal exists with four categories: Brush Tip, Dynamics, Color, Texture.
BrushConfig (in hcie-protocol/src/tools.rs) exposes only size, opacity, hardness, flow, spacing, style, blend_mode, cyclic_color, cyclic_speed, color_variant, variant_amount, density.
Engine BrushTip already has jitter_amount, scatter_amount, angle, roundness, but the GUI never sets them (canvas code hard-codes 0.0 / 1.0).
Preview is procedural (brushes_panel::draw_brush_preview) and does not reflect actual engine output.
Dock panels vary: some use ScrollArea, some do not; scrollbars are visible.

Decisions

1. New brush dynamics fields

Add to hcie-protocol/src/tools.rs BrushConfig:

pub jitter_amount: f32,
pub scatter_amount: f32,
pub angle: f32,
pub roundness: f32,
pub drawing_angle: bool,
pub rotation_random: f32,
pub time_enabled: bool,
pub time_size_start: f32,
pub time_size_end: f32,
pub time_opacity_start: f32,
pub time_opacity_end: f32,
pub time_angle_start: f32,
pub time_angle_end: f32,

Defaults: 0.0, 0.0, 0.0, 1.0, false, 0.0, false, 1.0/1.0/1.0/1.0/0.0/0.0.

2. GUI controls

Brush Tip panel: keep Diameter, Hardness, Spacing, Style, Blend Mode; add Angle, Roundness, Drawing Angle checkbox, Rotation Random.
Dynamics panel: keep Opacity, Flow, Pressure Curve placeholder; add Jitter, Scatter.
Color panel: keep Color Variant, Cyclic Color; add a Time subsection with checkbox and start/end sliders for size, opacity, angle.
Every slider pairs with a right-aligned DragValue so mouse users can click/type exact values.
Angles displayed and edited in degrees, stored in radians.

3. Engine wiring

hcie-egui-app/crates/hcie-gui-egui/src/canvas/mod.rs: when building BrushTip, copy jitter_amount, scatter_amount, angle (converted to radians), roundness from state.tool_configs.brush.
hcie-brush-engine/src/lib.rs: update generate_brush_stamp and dab dispatchers so that:
- drawing_angle == true overrides angle with the stroke segment direction.
- rotation_random adds a per-dab random offset to the effective angle.
- roundness scales the stamp along the effective angle (elliptical dab).
hcie-engine-api/src/stroke_brush.rs: apply time_* interpolation using accumulated stroke distance (brush_accumulated_dist) before passing size/opacity/angle to the dab functions. Reset at stroke start (begin_stroke).

4. Real preview

Add Engine::render_brush_preview(&self, width, height, tip, color, stroke_points) -> Vec<u8> in hcie-engine-api/src/lib.rs.
It creates a temporary 1-layer document, draws the supplied stroke points, composites, and returns RGBA pixels.
brush_editor.rs calls this on every significant parameter change (when brush_editor_instant_preview is true), caches the texture, and draws it in the preview area instead of the procedural function.
Guard with a small debounce to avoid 4K-preview cost.

5. Scrollable panels

Create a helper scrollable_panel(ui, id, contents) in a new file hcie-egui-app/crates/hcie-gui-egui/src/app/scrollable_panel.rs.
It wraps egui::ScrollArea::vertical().always_show_scroll(false) and implements drag-to-pan by tracking pointer delta over the content rect when the left button is held and no widget is being dragged.
Apply the helper to every dock panel body:
- brushes_panel::show_brushes_ui
- panels.rs tool-property bodies (Tools, Layers, History, Properties, etc.)
- brush_editor.rs right-hand settings area
Keep existing ScrollArea calls but switch them to the helper so behavior is consistent.

Affected Boundaries (Locked Crates)

The following crates are normally locked by hcie-guard; the implementation agent must run unlock.sh <crate>, edit, then lock.sh <crate>:

hcie-protocol/src/tools.rs — BrushConfig fields
hcie-engine-api/src/lib.rs — new public preview API
hcie-engine-api/src/stroke_brush.rs — time interpolation
hcie-brush-engine/src/lib.rs — drawing angle / rotation / roundness behavior

GUI-only files (open to agent edits):

hcie-egui-app/crates/hcie-gui-egui/src/app/brush_editor.rs
hcie-egui-app/crates/hcie-gui-egui/src/app/brushes_panel.rs
hcie-egui-app/crates/hcie-gui-egui/src/app/panels.rs
hcie-egui-app/crates/hcie-gui-egui/src/app/mod.rs
hcie-egui-app/crates/hcie-gui-egui/src/canvas/mod.rs

Task Order

Extend BrushConfig in hcie-protocol and rebuild engine.
Implement engine preview API.
Implement drawing_angle, rotation_random, roundness in hcie-brush-engine.
Implement time interpolation in hcie-engine-api/src/stroke_brush.rs.
Wire new fields from GUI config to BrushTip in canvas/mod.rs.
Extend brush_editor.rs UI (new sliders, real preview, scrollable right panel).
Implement scrollable_panel helper and apply to all dock panels.
Run cargo test -p hcie-engine-api --test visual_regression and cargo test -p hcie-engine-api --test performance_stroke_4k.
Lock affected engine crates.
Capture screenshots of brush editor and each scrollable panel.

Validation

Visual regression tests (visual_regression) must still pass.
Performance regression benchmark (performance_stroke_4k) must not regress beyond 5%.
Brush editor preview updates within 200 ms of parameter change.
All dock panels scroll by mouse wheel and pan-drag without visible scrollbars.

Risks

hcie-brush-engine angle/roundness changes can alter default brush appearance if defaults are wrong; test with visual_regression.
Real preview allocates a temporary engine/document per frame; debounce/caching is mandatory.
Locked crate modifications require unlock.sh / lock.sh and possible terminal password prompt.

Open Questions

None remaining; user confirmed all decisions.

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