Plug-ins: filters read and set effects and stroke styles (G17)

A filter could reshape and repaint objects, but not set the stroke's
join, cap or miter limit, nor the effects an object's appearance holds.
A tool that prepares objects for a live effect (a sheet of samples,
each with the effect at its own settings) had to leave the user to
apply the effect to each object by hand.

- Each input stroke has its join, cap and miterLimit beside its paint
  and width (live effects get the same, through the shared encoder),
  and each object its own effect records, {id, params, visible}.
- An output stroke may set them (miter limit clamped to 1-500), and an
  output object's "effects" replaces its own effects.
- The host admits each record as effect.apply does: an effect that
  effect.list lists or an installed live-effect plug-in, parameters it
  takes (filled in to their defaults; a plug-in's checked against its
  manifest), one 3D effect that fits the object (its fills' and
  strokes' own effects counted), and the texture of an effect with
  Load Texture (Glass, Texturizer) as the effect stores it, never a
  file to read. A record the object already had stays as
  it is, so a filter that hands its input back changes nothing. One
  refused record refuses the whole output and the document stays as
  it was.

The plug-ins crate checks the records' form; which effects exist is the
engine's to say (objects::AdmitEffects, engine plugin::admit_effects).
effects::validate_three_d is the one check of whether a 3D effect fits
an object, for effect.apply, effect.setParams and the filters.

A path handed back as it was now stays the live shape it was: write()
dropped a live rectangle's or ellipse's parameters whenever the output
held a path, changed or not, so a filter that returned its input
turned a live rectangle into a plain path.

objects/tests.rs checks the round trip, stroke styles on existing and
new strokes, the host's refusal, the bad forms refused, and a live
rectangle handed back unchanged (a plain path before the fix);
tests_plugins.rs runs filters that set a built-in and a plug-in effect,
an echo that changes nothing, and six refused outputs.
CRAFT_PLUGIN_FRAME_WASM=<folder> saves a test filter to try in the app.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhvnnnD5fTFupK8WWq35un

docs/plugins.md

@@ -183,7 +183,9 @@ type and `blob` but without their data, which follows the JSON in a saved file;
    "strokes": [{"paint": {"type": "gradient", "gradient": {"kind": "Linear", "stops": [
        {"offset": 0, "color": {"model": "gray", "k": 0}},
        {"offset": 1, "color": {"model": "cmyk", "c": 0, "m": 0.5, "y": 1, "k": 0}}]}, "angle": 0},
-     "width": 2}],
+     "width": 2, "join": "round", "cap": "butt", "miterLimit": 10}],
+   "effects": [{"id": "distort.roughen", "params": {"detail": 10, "points": "smooth", "relative": true, "seed": 0,
+     "size": 5}, "visible": true}],
    "opacity": 1,
    "bounds": [10, 0, 75, 60]}
 ]}
@@ -192,12 +194,16 @@ type and `blob` but without their data, which follows the JSON in a saved file;
 - `type`: `"path"` or `"compound"` (a compound path: `path` holds its members' subpaths, painted as one).
 - `path`: VectorCraft's own path form. Each subpath is a run of anchors; `p` is the anchor, `in` / `out` its handles
   (absent: no handle), `kind` `"Smooth"` when the handles stay collinear. Coordinates are points, y down.
-- `fills`, `strokes`: the object's fills and strokes in paint order (bottom first); `null` is no paint. Paints use the
+- `fills`, `strokes`: the object's fills and strokes in paint order (bottom first); `null` is no paint. A stroke has its
+  `paint`, `width` in points, `join` (`"miter"`, `"round"` or `"bevel"`), `cap` (`"butt"`, `"round"` or `"square"`)
+  and `miterLimit`. Paints use the
   native file format's form: `{"type": "solid", "color", "swatch"?, "tint"?}`, `{"type": "gradient", "gradient":
   {"kind": "Linear"|"Radial"|"Freeform", "stops": [{offset, color, opacity, midpoint}]}, "geom"?, "angle",
   "freeform"?}`, `{"type": "pattern", "pattern", "xf"?}`. Colours are `{"model": "rgb", r, g, b}`, `{"model": "cmyk",
   c, m, y, k}`, `{"model": "gray", k}` (ink: 0 white, 1 black) or `{"model": "lab", l, a, b}`; components 0–1 except
   Lab's.
+- `effects` (filters): the object's own effects in order, as its effect records: `id` (as `effect.list` lists it, or
+  `plugin.<id>` for a live-effect plug-in), `params` and `visible`. Its fills' and strokes' own effects aren't listed.
 - `bounds`: `[x0, y0, x1, y1]`, the geometric bounds (read only).
 
 ### Parameters
@@ -226,17 +232,27 @@ For an **object filter**, the output is the new state of the input objects:
 
 - An object with the `id` of an input object updates it; keys it leaves out keep their value (`{"id": 12}` keeps the
   object as it is). `fills` and `strokes` set the object's fills and strokes in order: extra entries add fills (above
-  the last one) or strokes, missing ones are removed; each stroke keeps its weight unless `width` is given (a new one: 1 pt).
+  the last one) or strokes, missing ones are removed; each stroke keeps its weight, join, cap and miter limit unless
+  `width`, `join`, `cap` or `miterLimit` are given (a new one: 1 pt, miter join, butt cap, miter limit 10).
+- `effects` replaces the object's own effects, in order: each `{id, params?, visible?}`. A record the object already had
+  stays as it is; a new one is admitted as `effect.apply` admits it: an effect `effect.list` lists or an installed
+  live-effect plug-in, with parameters it takes (filled in to their defaults; a plug-in's checked against its
+  manifest), at most one 3D effect (its fills' and strokes' own effects counted), which must fit the object, and a
+  Glass `textureImage` as Glass stores it (base64 of a PNG: a plug-in can't have the host read a file). A tool can
+  then set up objects for a live effect in one step.
 - An object without an `id` is new and needs a `path`. It goes just above the object listed before it, or, listed
   before any input object, just below the first one listed after it. New objects take a path's defaults: no fills or
   strokes, full opacity.
 - Input objects missing from the output are deleted. The order of the input objects themselves doesn't change.
 - `type` turns a path into a compound path (one member per subpath) or back; `name: ""` clears the name.
+- A `path` returned as it was keeps a live shape live (a rectangle keeps its corner widgets); a changed one makes it a
+  plain path.
 
 The host validates everything before the document changes: coordinates must be finite and within ±4,000,000 pt, at
 most 100,000 objects and 4,000,000 anchors, 256 fills, strokes or gradient stops; colours, opacities and offsets are
-clamped into range, stroke weights to 0–1,000 pt; a pattern must exist in the document. A paint returned unchanged
-keeps its swatch link; any other paint is unlinked.
+clamped into range, stroke weights to 0–1,000 pt and miter limits to 1–500; a pattern must exist in the document. A
+paint returned unchanged keeps its swatch link; any other paint is unlinked. One refused effect record refuses the
+whole output.
 
 For a **live effect**, the geometry of every returned object's `path` (their subpaths, in order) replaces the
 object's; the rest is ignored (the object keeps its own appearance). An empty list removes the geometry.