feat(tests): caption posted screenshots (name + spec); raise drift floor to 0.5%

Bake a caption strip onto every posted screenshot composite — status + name +
the spec that produces it — for changed, added, and removed; removed now shows
the old baseline image (was a text-only line). Zero native-dep: an embedded
public-domain 8x8 bitmap font (font8x8.ts) rendered by image-ops `withBottomLabel`;
the name→spec attribution is factored out of gen-manifest into a shared
`spec-map.ts` resolver. Bottom strip, colour per status (green/red/orange).
Applies in both compare (drift gate) and changelog (git-history diff), and
post-discord now attaches the captioned removed images.

Also raise the per-engine drift floor 0.2% → 0.5% (changedRatio) to absorb the
sub-1% inter-run flakiness seen after the re-baseline, while still catching real
localized changes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/tools/screenshots/spec-map.ts

@@ -0,0 +1,81 @@
+/**
+ * Resolve a screenshot name → the spec that writes it (and the engine that renders
+ * it), by scanning the specs for `page.screenshot({ path: 'test-results/<prefix>…' })`
+ * literals. Shared by gen-manifest.ts (engine tags) and compare/changelog (caption
+ * labels).
+ *
+ * Attribution is longest-prefix (`pcbnew-loaded` → the spec that writes `pcbnew-…`).
+ * Names written only by a helper (e.g. `wizard-*` via completeWizard, which lives in
+ * a util not a *.spec.ts) don't match → specFor returns null (the caption then just
+ * shows the name).
+ */
+import * as fs from 'fs';
+import * as path from 'path';
+
+export const CHROMIUM = 'chromium-swiftshader';
+export const FIREFOX = 'firefox-llvmpipe';
+export const DEFAULT_ENGINE = CHROMIUM; // baseline-screenshots is dominated by the wx suite
+
+function listSpecs(dir: string): string[] {
+    const out: string[] = [];
+    if (!fs.existsSync(dir)) return out;
+    for (const e of fs.readdirSync(dir, { withFileTypes: true })) {
+        const p = path.join(dir, e.name);
+        if (e.isDirectory()) out.push(...listSpecs(p));
+        // Only *.spec.ts — a literal in a util would be mis-attributed to the util's dir.
+        else if (e.name.endsWith('.spec.ts')) out.push(p);
+    }
+    return out;
+}
+
+/** pcbnew-family spec basenames (routed to chromium-ci on CI), read from the config. */
+function pcbnewFamily(root: string): Set<string> {
+    const cfg = fs.readFileSync(path.join(root, 'playwright-kicad.config.ts'), 'utf8');
+    const block = cfg.match(/PCBNEW_FAMILY_SPECS\s*=\s*\[([\s\S]*?)\]/)?.[1] ?? '';
+    return new Set([...block.matchAll(/'[^']*?([\w.-]+\.spec\.ts)'/g)].map((m) => m[1]));
+}
+
+function engineForSpec(root: string, specPath: string, family: Set<string>): string {
+    const rel = path.relative(root, specPath);
+    const base = path.basename(specPath);
+    if (rel.startsWith('e2e/')) return CHROMIUM;
+    if (rel.startsWith('web/')) return FIREFOX;
+    if (rel.startsWith('kicad/')) return family.has(base) ? CHROMIUM : FIREFOX;
+    return DEFAULT_ENGINE;
+}
+
+type PrefixEntry = { prefix: string; engine: string; spec: string };
+
+/** prefix → {engine, spec-rel-to-tests}, longest-prefix first. */
+function prefixMap(root: string, family: Set<string>): PrefixEntry[] {
+    const map = new Map<string, { engine: string; spec: string }>();
+    for (const dir of ['e2e', 'kicad', 'web']) {
+        for (const spec of listSpecs(path.join(root, dir))) {
+            const engine = engineForSpec(root, spec, family);
+            const rel = path.relative(root, spec); // e.g. 'kicad/pcbnew.spec.ts'
+            const content = fs.readFileSync(spec, 'utf8');
+            for (const m of content.matchAll(/test-results\/([A-Za-z0-9_-]+)/g)) {
+                const prefix = m[1];
+                if (!map.has(prefix)) map.set(prefix, { engine, spec: rel }); // first writer wins
+            }
+        }
+    }
+    return [...map.entries()]
+        .map(([prefix, v]) => ({ prefix, ...v }))
+        .sort((a, b) => b.prefix.length - a.prefix.length);
+}
+
+export type SpecResolver = { specFor(name: string): string | null; engineFor(name: string): string };
+
+/** Build a name→spec/engine resolver by scanning the specs under `root` once. */
+export function buildSpecResolver(root: string): SpecResolver {
+    const prefixes = prefixMap(root, pcbnewFamily(root));
+    const match = (name: string): PrefixEntry | undefined => {
+        const stem = name.replace(/\.png$/i, '');
+        return prefixes.find((p) => stem === p.prefix || stem.startsWith(p.prefix));
+    };
+    return {
+        specFor: (name) => match(name)?.spec ?? null,
+        engineFor: (name) => match(name)?.engine ?? DEFAULT_ENGINE,
+    };
+}