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/0008-tsdown-over-dumble.md

@@ -0,0 +1,52 @@
+# ADR 0008: tsdown for JS bundling instead of dumble
+
+Status: accepted (2026-06-11)
+
+## Context
+
+The initial build used **dumble**, the cordiverse zero-config esbuild wrapper
+that upstream Cordis itself builds with — maximum alignment with the vendored
+packages' conventions (it reads each package.json and infers entries/formats
+from the `exports` field). But dumble is a liability as a load-bearing tool in
+this repo: v0.2.x, ~530 npm downloads/week, effectively one maintainer, and we
+were invoking it through a custom orchestration script (`scripts/build.ts`)
+because it has no workspace mode.
+
+Build output currently matters only for `yarn build` + publint (nothing
+publishes yet; dev/test/demo run unbuilt via tsx), so the switching cost is at
+its lowest now and only grows once packages publish.
+
+## Decision
+
+Replace dumble with **tsdown** (rolldown-based, ~2.5M downloads/week,
+VoidZero-backed, actively released):
+
+- Root `tsdown.config.ts` with `workspace: ['vendor/*', 'packages/*']`
+  (explicit globs, not `workspace: true`, which would also pick up
+  `examples/*` — they have package.json files but are not yarn workspaces).
+- Shared shape: entry `src/index.ts`, `outDir: 'lib'`, ESM, `platform: node`,
+  `target: es2024`, `fixedExtension: false` (keeps `.js` for
+  `"type": "module"` packages), `dts: false` (tsc -b owns declarations),
+  `clean: false` (lib/ holds tsc's .d.ts output).
+- Two per-package overrides in vendor/ (ours, like the regenerated tsconfigs;
+  logged in vendor/README.md): schemastery (dual `.mjs`/`.cjs` via
+  `outExtensions`), logger-console (two single-entry passes so the shared
+  base class is inlined into each entry instead of a hash-named chunk,
+  matching upstream's published shape).
+- `scripts/build.ts` deleted; `yarn build` = `tsc -b && tsdown`.
+
+Alternatives considered: **direct esbuild script** (most established engine,
+zero wrapper risk, but hand-maintains the per-package spec table tsdown's
+workspace mode gives us); **pkgroll** (closest drop-in philosophically, but
+78k dl/wk and Rollup-based — strictly weaker maintenance story than tsdown);
+**keep dumble** (perfect upstream alignment, unacceptable bus factor).
+
+## Consequences
+
+Output file lists are byte-for-byte-list identical to dumble's (verified by
+snapshot diff at migration time); externals still come from each package's
+dependencies/peerDependencies. We give up dumble's exports-field inference —
+new packages with non-default shapes need a per-package `tsdown.config.ts`
+instead of just package.json fields. Future option: tsdown could also absorb
+declaration bundling (isolatedDeclarations) if `tsc -b` ever becomes the
+bottleneck; that would be a new ADR.

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 |