fix(diagrams): #104 review — an emoji is measured as wide as it was meant to be; a dead store goes

Two findings from the code-quality review. Both are right.

theme.js, charEm() — "the condition c > 0xffff is always false". The width
estimate used where there is no canvas asked "CJK and other full-width scripts"
(U+2E80 and up, 1.0 em) before "emoji / astral" (above U+FFFF, 1.1 em). Every
code point above U+FFFF is above U+2E80 too, so the second question was never
reached and an emoji was measured at 1.0 em. The estimate is meant to err wide
— the failure that matters is text overflowing its box — so the questions are
now asked the other way round.

Nothing on screen changes. The chat measures with the real font, and no fixture
contains a character beyond the basic plane, so no snapshot moves. What changes
is the fallback when the canvas refuses the font, and the estimates made in
Node.

layout.js — "the value assigned to span here is unused". When several
connectors share a side too short for them, the node grows to hold exactly what
they need; span was recomputed afterwards and never read. The recompute is
gone and span is a const. it.cs, set on the same line, IS read later, and
stays. 6,000 random layouts and the nine gallery diagrams are byte-identical to
the code before.

diagramLayout gains a test for the estimate's classes: narrow, ordinary and
wide letters, digits and capitals, a CJK character at a full em, an emoji at
1.1 em and counted as one character. It fails on the old order, on exactly the
condition the review named.

62 suite files pass on macOS and in a Linux container.

Author: ndemianc

docs/RICH-DIAGRAMS.md

@@ -391,7 +391,7 @@ on the default models, then use it for a week and read the counters.
 
 Verified:
 
-- 12 suites, 251 tests (`test/diagram*.test.js`), including a 1,500-spec layout fuzz, the corpus,
+- 12 suites, 252 tests (`test/diagram*.test.js`), including a 1,500-spec layout fuzz, the corpus,
   SVG snapshots in both palettes, the gallery at chat-panel widths, the real `runAgent` loop with a
   scripted provider, and the host functions sliced out of `extension.js` against a `vscode`
   stand-in.

extensions/levelcode-ai/diagram/layout.js

@@ -381,15 +381,15 @@
 				const lane = S.labelLane != null ? S.labelLane : T.edge.line + S.labelGap + 2;
 				const floor = labelled ? lane : (S.portMin != null ? S.portMin : 6);
 				let gap = Math.max(S.portGap, labelled ? floor : 0);
-				let span = portSpan(it.node, it.cs, S);
+				const span = portSpan(it.node, it.cs, S);
 				if (n > 1 && (n - 1) * gap > span) {
 					gap = Math.max(floor, span / (n - 1));
 					if ((n - 1) * gap > span + 1e-6) {
 						// Still does not fit: the node grows along its side. ("Renderer only: widen, wrap.")
 						const need = (n - 1) * gap;
 						const grow = it.node.shape === 'decision' ? need / 0.5 : it.node.shape === 'actor' ? need / 0.55 : need + 2 * S.portInset;
 						if (right) { it.node.h = Math.ceil(grow); } else { it.node.w = Math.ceil(grow); }
-						it.cs = cs(it.node); span = portSpan(it.node, it.cs, S);
+						it.cs = cs(it.node);   // (the node now holds exactly `need`: there is nothing left to re-measure)
 					}
 				}
 				ends.forEach((en, i) => {

extensions/levelcode-ai/diagram/theme.js

@@ -178,8 +178,8 @@
 		const c = ch.codePointAt(0) || 0;
 		if (c >= 0x30 && c <= 0x39) { return 0.60; }          // digits
 		if (c >= 0x41 && c <= 0x5a) { return 0.69; }          // A–Z
+		if (c > 0xffff) { return 1.1; }                       // emoji and everything else beyond the basic plane — asked first: all of it is above U+2E80 too
 		if (c >= 0x2e80) { return 1.0; }                      // CJK and other full-width scripts
-		if (c > 0xffff) { return 1.1; }                       // emoji / astral
 		return 0.57;
 	}
 	/**

extensions/levelcode-ai/test/diagramLayout.test.js

@@ -305,6 +305,19 @@ test('PORTS: connectors that share a side each get their own point on it — and
 	assert.ok(byId(big, 'h').h > byId(big, 't0').h, 'the hub grew along its side');
 });
 
+test('MEASURE: the estimate used where there is no canvas errs WIDE, class by class — an emoji is wider than a CJK character', () => {
+	// Regular weight at 11.5px, so the numbers are plain multiples of the em.
+	const px = (t) => theme.approxMeasure(t, 'sub');
+	assert.ok(px('i') < px('a') && px('a') < px('m'), 'narrow, ordinary and wide letters');
+	assert.ok(px('7') > px('a') && px('A') > px('7'), 'digits and capitals are wider than lower case');
+	assert.strictEqual(px('\u65e5'), 11.5, 'a CJK character is a full em');
+	assert.strictEqual(px('\u{1f680}'), 12.7, 'an emoji is 1.1 em — the widest class, not lumped in with CJK');
+	assert.ok(px('\u{1f680}') > px('\u65e5'));
+	assert.strictEqual(px('\u{1f680}\u{1f680}'), 25.3, 'and it is ONE character, though it is two code units');
+	assert.strictEqual(px('\u{20000}'), 12.7, 'anything else beyond the basic plane is measured the same generous way');
+	assert.strictEqual(px(''), 0); assert.strictEqual(px(null), 0);
+});
+
 test('SHAPES: size comes from the measured text — and a connector stops ON a diamond\'s outline', () => {
 	const double = (t, role) => theme.approxMeasure(t, role) * 2;
 	const s = R.prepare({ title: 'T', nodes: [{ id: 'a', label: 'A fairly long label' }], edges: [] }).spec;