0541 — examples, bench, and reference become dev-only nested modules

Context

The roadmap carried “go.mod cleanup after factoring examples into a separate module,” premised on examples weighing down the root go.mod. Measured, the premise was empty: examples/bench/reference import only pgx and uuid, and pkg/ itself needs all three direct requires (pgx, uuid, x/sync) — go mod tidy on the root after the split is a no-op. The real costs of keeping them in the root module were elsewhere: ~40 lab mains plus benchmark and reference scratch shipped in the published module zip, and reference/waterline’s tests ran in the root go test ./....

Decision

Carve each dev-only tree into its own nested module, on the existing cmd/vulkan / otelvulkan pattern:

examples/go.mod, bench/go.mod, reference/go.mod; the repo-root go.work gains use lines for all three.
No require line for the parent module (unpublished; a placeholder version poisons the workspace graph — same rule as cmd/vulkan).
Unlike cmd/vulkan and otelvulkan, these three are NEVER tagged or published, so they never take a pinned parent require at release. The release story stays three modules: root, cmd/vulkan, otelvulkan.

Consequences

A directory with its own go.mod is excluded from the parent module’s zip: go get github.com/agentstax/vulkan downloads the library only.
Root go test ./... and go list ./... no longer include examples/bench/reference packages (65 tests, 70 packages after).
justfile lab recipes are untouched: go run examples/phase_1/... from the repo root resolves through the workspace.
Building examples requires the workspace; there is no module-only build of the labs against a published parent version, by design.
The roadmap’s go.mod-cleanup follow-up dissolves: the root go.mod was already exactly the library’s needs.

Directory naming was superseded by [0720] and [0723]. The dev-only module boundary remains in force.