build(8.0): clean dist before every build so stale artifacts never ship

The incremental `tsc` build never removed dist output for source files
deleted in earlier refactors, so a cluster of 5-month-old artifacts kept
shipping in the published package — an entire orphaned native module tree
(dist/native/*, the Native* graph/metadata adapters, the mmap adapter)
whose source was removed when those responsibilities moved behind the
provider boundary. Nothing in the live tree imported them; they were dead
weight in every tarball, carrying stale type signatures with no source.

Add a `clean` script (cross-platform `fs.rmSync`) and a `prebuild` hook so
every build — including the `prepare` build that runs on publish — starts
from an empty dist. Verified: a planted stale file is wiped on the next
build, and a clean rebuild drops the entire orphaned tree.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -66,6 +66,8 @@
     "bun": ">=1.0.0"
   },
   "scripts": {
+    "clean": "node -e \"require('fs').rmSync('dist',{recursive:true,force:true})\"",
+    "prebuild": "npm run clean",
     "build": "npm run build:types:if-needed && npm run build:patterns:if-needed && tsc && tsc -p tsconfig.cli.json && npm run build:copy-wasm",
     "build:copy-wasm": "node -e \"const fs=require('fs');const src='src/embeddings/wasm/pkg';const dst='dist/embeddings/wasm/pkg';const skip=new Set(['package.json','.gitignore']);if(fs.existsSync(src)){fs.mkdirSync(dst,{recursive:true});fs.readdirSync(src).filter(f=>!skip.has(f)).forEach(f=>fs.copyFileSync(src+'/'+f,dst+'/'+f));console.log('Copied WASM pkg to dist')}\"",
     "build:types": "tsx scripts/buildTypeEmbeddings.ts",