Replace dumble with tsdown for JS bundling

dumble (0.2.x, ~530 dl/wk, single-maintainer) was a bus-factor risk as
the load-bearing bundler. tsdown (rolldown-based, ~2.5M dl/wk, actively
maintained) replaces it while output stays list-identical, verified by
snapshot diff: 17 JS bundles, externals preserved, schemastery dual
.mjs/.cjs and logger-console node+browser entries intact.

Root tsdown.config.ts uses workspace globs ['vendor/*', 'packages/*']
(explicit, so examples/* stays excluded); two per-package overrides in
vendor/ cover the special shapes and are logged in vendor/README.md as
ours (not upstream sync surface). scripts/build.ts (dumble
orchestration) is deleted; yarn build = tsc -b && tsdown. tsc -b keeps
owning declarations (dts: false, clean: false).

Rationale recorded in ADR 0008 (also covers the direct-esbuild and
pkgroll alternatives).

Gates: lint, typecheck, 134 tests, hygiene (knip/publint/constraints),
demo smoke all green.

vendor/schemastery/tsdown.config.ts

@@ -0,0 +1,18 @@
+import { defineConfig } from 'tsdown'
+
+/**
+ * schemastery has no `"type": "module"` and publishes dual-format output
+ * (package.json: main → lib/index.cjs, module → lib/index.mjs). Pin the
+ * extensions explicitly — the defaults for a CommonJS package would emit
+ * .mjs/.js instead.
+ */
+export default defineConfig({
+  entry: ['src/index.ts'],
+  outDir: 'lib',
+  format: ['esm', 'cjs'],
+  platform: 'node',
+  target: 'es2024',
+  outExtensions: ({ format }) => ({ js: format === 'es' ? '.mjs' : '.cjs' }),
+  dts: false,
+  clean: false,
+})