Plug-ins: labels, units, groups, help and conditions for parameters (G17)

A plug-in's dialog labelled each field with its parameter's name spaced
out ("Line_spacing_mm:"), showed numbers without their unit and listed
at most 64 parameters in one column. A plug-in with many settings, such
as a generator's or an exporter's, could neither explain nor sort them,
and its dialog showed settings that don't apply to the choices made.

Each parameter may now say how dialogs show it, in keys that hosts
from before them ignore (plugins::dialog):

- label; unit, after the value ("0.25 mm"); help, on the label; and
  helpUrl, an https:// page opened from a button after the field.
- group: the parameters outside any group come first, then each group
  as a section that folds, in the order the groups first appear, the
  fields lined up down the dialog.
- visibleWhen: {"pattern": ["lines"]} shows a parameter only
  while each checkbox or choice it names, declared before it, has one
  of the values listed and is shown itself. Manifest::visible decides
  every parameter in one pass, in declaration order. A hidden parameter
  still goes to the plug-in with its value, so a run never depends on
  what the dialog showed.

Two types join number, int, bool and choice: string, a text field cut
to maxLength, and numberList, numbers typed separated by commas or
spaces, each clamped, minItems to maxItems of them. A plug-in declares
up to 256 parameters, at most 64 outside groups. The filter, live-effect
and exporter dialogs use every key. The Appearance panel's editor of a
plug-in effect uses the labels, units, help and conditions, and reads a
typed number list as the dialog does.

dialog.rs tests parsing, the round trip of a manifest written out,
sections, visibility (also of 256 overlapping conditions, which a
recursive check would take forever on) and the keys refused;
manifest.rs the new types and limits. tests_plugin_fields.rs draws a
dialog with kittest: labels and units, sections in order with their
fields lined up, visibleWhen hiding and showing rows, and the help
button opening its page.
appearance.rs checks the editor's labels, units, conditions and a
typed number list, and form.rs units after numbers (a unit starting
with a digit is spaced from it) and typed lists.

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

# Conflicts:
#	docs/gaps.md
#	docs/target-app-parity.md

docs/plugins.md

@@ -39,9 +39,10 @@ its extensions that no built-in format has; `selectedOnly` hands it the selectio
 
 In the app, installed filters appear under **Object › Plug-ins**, next to **Install Plug-in…** and **Reload
 Plug-ins**; live effects under **Effect › Plug-ins** and in the Appearance panel's effect menu. A plug-in with
-parameters gets a dialog generated from its schema (numbers, whole numbers, checkboxes, dropdowns) with a live
-preview; without parameters it runs at once. Parameters too many for the window's height scroll inside the dialog,
-which keeps its heading and buttons in view. Exporters are formats of **File › Export › Export As**, where Use
+parameters gets a dialog generated from its schema (numbers, whole numbers, checkboxes, dropdowns, text and lists
+of numbers, labelled, explained and sorted into sections as its manifest says) with a live preview; without
+parameters it runs at once. Parameters too many for the window's height scroll inside the dialog, which keeps its
+heading and buttons in view. Exporters are formats of **File › Export › Export As**, where Use
 Artboards is greyed out (the plug-in writes the whole document); one with parameters asks for them in a dialog of the
 same kind once the file is picked, and its Export button writes the file. **File › Open** of a `.wasm` file installs
 a plug-in too (on the web this is how plug-ins get in).
@@ -111,10 +112,11 @@ pointers are offsets into the module's memory.
   "author": "Example",
   "description": "What it does.",
   "params": {
-    "amount": {"type": "number", "min": 0, "max": 100, "default": 50},
-    "steps":  {"type": "int", "min": 1, "max": 16, "default": 4},
+    "amount": {"type": "number", "min": 0, "max": 100, "default": 50, "unit": "%"},
+    "steps":  {"type": "int", "min": 1, "max": 16, "default": 4, "label": "Steps per curve"},
     "closed": {"type": "bool", "default": false},
-    "mode":   {"type": "choice", "options": ["soft", "hard"], "default": "soft"}
+    "mode":   {"type": "choice", "options": ["soft", "hard"], "default": "soft",
+               "help": "Soft keeps the corners round; hard keeps them sharp."}
   }
 }
 ```
@@ -130,7 +132,28 @@ pointers are offsets into the module's memory.
 - `params`: in dialog order. Names are `[A-Za-z0-9_]`, not starting with `_`, and not `id`, `ids`, `params` or
   `preview`. The host validates the user's values before calling: missing values take the default, numbers are
   clamped to `min..max` (ints rounded), a wrong type or an unknown choice is an error. Numeric defaults
-  are also clamped to their range, including the implicit zero default of an `int` parameter.
+  are also clamped to their range, including the implicit zero default of an `int` parameter. Besides `number`,
+  `int`, `bool` and `choice` there are `string` (`{"type": "string", "default": "", "maxLength": 256}`: a text field;
+  longer text is cut to `maxLength`, at most 4,096 characters) and `numberList` (`{"type": "numberList", "min": 0,
+  "max": 360, "default": [45, 135], "minItems": 1, "maxItems": 16}`: numbers typed separated by commas or spaces,
+  each clamped to `min..max`; a list of too few or too many is an error, and `maxItems` is at most 256). A plug-in
+  declares up to 256 parameters, at most 64 of them outside groups.
+
+Each parameter may also say how dialogs show it. Hosts from before these keys ignore them, so a plug-in that
+declares them still installs there:
+
+| Key | What the dialog does with it |
+|---|---|
+| `label` | Labels the field (1–64 characters). Without one, the label is the name spaced out (`rowSpacing` → Row Spacing). |
+| `unit` | Shows the unit the value is in after it (1–16 characters, such as `mm` or `°`). Nothing is converted: the plug-in gets the number as typed. |
+| `group` | Puts the parameter in a section that folds (1–64 characters). Parameters outside any group come first, then each group in the order it first appears. |
+| `help` | Shows what the parameter does when the pointer rests on its label (1–1,000 characters; line breaks allowed). |
+| `helpUrl` | Opens this `https://` page (at most 512 bytes) from a button after the field. |
+| `visibleWhen` | `{"pattern": ["lines", "crosshatch"], "outline": [true]}` shows the parameter only while each parameter it names (a checkbox or a choice declared before it) has one of the values listed and is shown itself. |
+
+A hidden parameter still goes to the plug-in with its value: what the dialog showed never changes a run. The
+filter, live-effect and exporter dialogs use all of these keys. The Appearance panel's editor of a plug-in effect's
+parameters uses `label`, `unit`, `help` and `visibleWhen`, and lists the parameters without sections or help buttons.
 
 ### Input