🔧 Clean dist before each build

tsc does not prune orphaned emit output, so dropping declarationMap left
stale *.d.ts.map files in dist/ until a manual clean. A prebuild hook now
empties dist/ first, keeping the build reproducible. It deliberately leaves
coverage/ alone, since the manual `clean` resets both.

package.json

@@ -38,6 +38,7 @@
     },
     "scripts": {
         "build": "tsc -p tsconfig.build.json",
+        "prebuild": "rm -rf dist",
         "check": "npm run check:tsc && npm run check:oxlint && npm run check:oxfmt && npm run check:cspell",
         "check:cspell": "cspell lint ${LEFTHOOK_FILES:-.}",
         "check:oxfmt": "oxfmt --check ${LEFTHOOK_FILES:-.}",