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/logger-console/tsdown.config.ts

@@ -0,0 +1,23 @@
+import { defineConfig } from 'tsdown'
+
+/**
+ * logger-console ships two entries: the node exporter (index) and the
+ * browser exporter (browser), selected via package.json `exports`
+ * conditions. They are built as two single-entry passes so the shared
+ * base class is inlined into each (matching upstream's published shape)
+ * instead of split into a hash-named chunk.
+ */
+const shared = {
+  outDir: 'lib',
+  format: ['esm'],
+  platform: 'node',
+  target: 'es2024',
+  fixedExtension: false,
+  dts: false,
+  clean: false,
+} as const
+
+export default defineConfig([
+  { ...shared, entry: ['src/index.ts'] },
+  { ...shared, entry: ['src/browser.ts'] },
+])