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/README.md

@@ -49,6 +49,11 @@ Keep this log exhaustive — every divergence from upstream must be listed.
    peer-dependency ranges preserved.
 3. **All `tsconfig.json` files**: regenerated to extend the repo-root
    `tsconfig.base.json` and declare project references.
+4. **`schemastery/tsdown.config.ts` and `logger-console/tsdown.config.ts`**:
+   ours, not upstream files — per-package build-shape overrides (dual
+   ESM+CJS output; separate node/browser entries) for the repo-root tsdown
+   build. Like the regenerated tsconfigs, they are not part of the upstream
+   sync surface.
 
 ## Sync procedure