Check the canvas size before narrowing it to an int

A canvas side was cast to int with no range check, so a number that did
not fit came back as one that did: `-s 4294968296x600` rendered a
perfectly plausible 1000x600, and a hand-edited project file could ask
for a width the reader turned into something else (or, for a double out
of range, into undefined behaviour).

Both front-ends now check the side before the cast, against one shared
bound in chain.hpp — the 1..20000 the GUI spin buttons already offered,
so the help text, the CLI error, the project reader and the interface
all move together.

The tests gain a `canvas size` section covering the round trip at both
ends of the range and the refusals on either side of it, through the
project reader and through `-s`; the test binary picks up the CLI parser
to do it, which stays GUI-free.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/cli/options.cpp

@@ -24,7 +24,10 @@ bool parseSize(const std::string& text, int& width, int& height) {
     long w = 0;
     long h = 0;
     if (!parseLong(text.substr(0, sep), w) || !parseLong(text.substr(sep + 1), h)) return false;
-    if (w <= 0 || h <= 0) return false;
+    // Checked as longs, before the narrowing: 4294968296 cast straight to int
+    // is 1000, which would render a plausible canvas nobody asked for.
+    if (w < kMinCanvasSide || w > kMaxCanvasSide) return false;
+    if (h < kMinCanvasSide || h > kMaxCanvasSide) return false;
     width = static_cast<int>(w);
     height = static_cast<int>(h);
     return true;
@@ -66,7 +69,9 @@ Options parseOptions(int argc, char** argv) {
         } else if (arg == "-s" || arg == "--size") {
             const std::string value = requireValue(argc, argv, i, arg);
             if (!parseSize(value, options.width, options.height)) {
-                throw EffectError("invalid size '" + value + "', expected e.g. 1920x1080");
+                throw EffectError("invalid size '" + value + "', expected e.g. 1920x1080 with "
+                                  "each side between " + std::to_string(kMinCanvasSide) + " and " +
+                                  std::to_string(kMaxCanvasSide));
             }
         } else if (arg == "--seed") {
             const std::string value = requireValue(argc, argv, i, arg);
@@ -131,7 +136,8 @@ std::string usage(const char* programName) {
         << "Options:\n"
         << "  -e, --effect NAME     effect to apply, repeat to cascade (default: " << defaultEffectName() << ")\n"
         << "  -o, --output FILE     render headlessly to this file and exit\n"
-        << "  -s, --size WxH        canvas size (default: 1920x1080)\n"
+        << "  -s, --size WxH        canvas size, each side " << kMinCanvasSide << "-"
+        << kMaxCanvasSide << " (default: 1920x1080)\n"
         << "      --seed N          random seed (default: random)\n"
         << "      --list-effects    list available effects and exit\n"
         << "  -h, --help            show this help and exit\n";