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.

docs/adr/README.md

@@ -17,3 +17,4 @@ with a new one and cross-link.
 | [0005](0005-custom-schema-dsl-over-schemastery.md) | Custom typed tool-schema DSL instead of schemastery | accepted |
 | [0006](0006-tool-schemas-in-prompt-assembly.md) | Tool schemas are part of the system-prompt assembly | accepted |
 | [0007](0007-quality-gates.md) | Mechanical quality gates over prose guidelines | accepted |
+| [0008](0008-tsdown-over-dumble.md) | tsdown for JS bundling instead of dumble | accepted |