Plot styles: named styles (STB), PLOTSTYLE, STYLESMANAGER and table files (part of #410) (#418)

Layers and objects get a PlotStyle property (ByLayer/ByBlock/name) applied
through named tables (built-in default.stb and monochrome.stb, our own
definitions). DXF keeps the names in the ACAD_PLOTSTYLENAME dictionary of
ACDBPLACEHOLDER objects with group 390 on layers and objects; tables kept in
the drawing travel in a CADCRAFT_PLOTSTYLES XRECORD. PLOTSTYLE sets an object's
or the current (CPLOTSTYLE) style; STYLESMANAGER lists, creates, edits, deletes,
loads and saves tables in our own JSON format (docs/plot-styles.md). A Plot
Style Manager dialog edits a table's styles and the layout's table; the
Properties palette shows the current plot style and table.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/plot-styles.md

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+# Plot styles
+
+A plot style table decides how objects print. CADCraft applies it to PDF plots (PLOT,
+EXPORTPDF and anything that plots through them). It also applies it to a layout shown with
+**Display plot styles** on: on screen and in the layout's PNG and SVG exports.
+
+## Tables
+
+| Kind | Picks the style by | Styles |
+|---|---|---|
+| Colour-dependent (`.ctb`) | the object's index colour (a true colour uses its nearest index colour) | exactly 255, `Color_1` … `Color_255` |
+| Named (`.stb`) | the object's **PlotStyle** property | `Normal` first, then any names |
+
+A style can set these:
+
+| Field | Meaning | "Use object" value |
+|---|---|---|
+| `color` | output colour, `"#rrggbb"` | absent / `null` |
+| `grayscale` | print the colour as the grey of its luminance (Rec. 709 weights) | `false` |
+| `screening` | ink intensity 0–100 %, mixing toward the white paper | `100` |
+| `linetype` | linetype name (loaded, or from the built-in library; `Continuous` draws solid) | absent / `null` |
+| `lineweight` | hundredths of a millimetre, 0–211; used when lineweights are plotted | absent / `null` |
+| `end`, `join`, `fill` | line end, line join and area fill style | `object` |
+
+Colour 7 prints black. `end`, `join` and `fill` are stored but not applied yet.
+
+### Built-in tables
+
+These are CADCraft's own definitions, not copies of any vendor's files.
+
+| Table | What it does |
+|---|---|
+| `default.ctb` | keeps every object property |
+| `monochrome.ctb` | prints every colour black |
+| `grayscale.ctb` | prints every colour as the grey of its luminance |
+| `default.stb` | `Normal` (object properties), `Black`, `Screened 50%`, `Thick` (0.50 mm) |
+| `monochrome.stb` | `Normal` prints black; `Screened 50%` prints black at 50 % |
+
+A table kept in the drawing shadows a built-in table of the same name.
+
+## Named plot styles
+
+Layers have a plot style name (default `Normal`). Objects have `ByLayer` (the default), `ByBlock`
+or a style name. `ByLayer` takes the layer's style. On layer 0 inside a block, that is the block
+reference's layer. `ByBlock` takes the block reference's style. A name the table doesn't have
+prints as `Normal`. A colour-dependent table ignores the property.
+
+## Commands
+
+- `pagesetup {layout, plotStyleTable, displayPlotStyles}` sets the layout's table and the
+  Display plot styles flag. Without a layout, it reports the built-in tables in `plotStyleTables`.
+- `plot` / `exportpdf {plotStyleTable}` replaces the page setup's table for one plot. `"None"`
+  plots object colours.
+- `plotstyle {name, handles?}` sets the objects' plot style. With nothing selected, it sets the
+  current plot style for new objects (`CPLOTSTYLE`). Without `name`, it reports the current
+  style and the named styles.
+- `layer.set {name, plotStyle}` sets a layer's plot style.
+- `stylesmanager {action, …}` manages the tables: `list`, `get`, `new` (`kind`, `from`), `set`
+  (`style: {name, …fields}`, `removeStyle`, `description`), `delete`, `load` (`path` or `text`)
+  and `save` (`path`, or returns `text`). Typed or chosen from **File ▸ Plot Styles...**, it
+  opens the Plot Style Manager dialog.
+
+## Table files
+
+Tables are saved in our own format: UTF-8 JSON with a format marker. Fields left at "use the
+object's" are omitted.
+
+```json
+{
+  "cadcraftPlotStyleTable": 1,
+  "name": "office.stb",
+  "description": "Office pens",
+  "kind": "named",
+  "styles": [
+    { "name": "Normal" },
+    { "name": "Pen 5", "color": "#000000", "screening": 40, "lineweight": 35, "linetype": "HIDDEN", "end": "round" }
+  ]
+}
+```
+
+- `kind` is `colorDependent` or `named`.
+- `end` is one of `object`, `butt`, `square`, `round` or `diamond`.
+- `join` is one of `object`, `miter`, `bevel`, `round` or `diamond`.
+- `fill` is one of `object`, `solid`, `checkerboard`, `crosshatch`, `diamonds`, `horizontalBars`,
+  `slantLeft`, `slantRight`, `squareDots` or `verticalBars`.
+- `color` may also be written `[r, g, b]`.
+
+Reading fixes hostile values:
+
+- Screening is capped at 100 and lineweights at 211.
+- A colour-dependent table always gets 255 styles named `Color_n`.
+- A named table gets unique style names (at most 1024), with `Normal` first.
+- Files over 4 MB are refused.
+
+Binary `.ctb`/`.stb` files are not read. We haven't found a public specification of that format.
+
+## In DXF files
+
+- The page setup's table is PLOTSETTINGS group 7, and Display plot styles is plot layout flag 2
+  (group 70).
+- Plot style names follow the DXF Reference. The named object dictionary entry
+  `ACAD_PLOTSTYLENAME` is an `ACDBDICTIONARYWDFLT` whose entries name `ACDBPLACEHOLDER` objects
+  (default `Normal`). Layers and objects point at them with group 390. An object without a 390
+  is `ByLayer`. `ByBlock` isn't written, so it reads back as `ByLayer`.
+- Tables kept in the drawing travel in the `CADCRAFT_PLOTSTYLES` XRECORD (JSON) of the named
+  object dictionary.