Viewer demo staging silently skips synthetic-mvp when gen-fixture is not built

[DrunkOnJava]

Problem

npm run build in viewer/ runs scripts/stage-demos.mjs (via prebuild). The synthetic-mvp demo is generated on the fly by the gen-fixture binary, but the script only looks for it on PATH or in target/. On a fresh clone it prints

synthetic-mvp: missing (gen-fixture not found — build with cargo build --release --bin gen-fixture)

and the gallery marks the demo unavailable. viewer/README.md describes the tier-1 demos being staged from corpus/tier1/ but never says that synthetic-mvp needs cargo build --release --bin gen-fixture first, so the warning is the first time a newcomer hears about it.

What to do

1. Document the prerequisite in viewer/README.md next to the npm run stage:demos mention (one or two sentences and the exact cargo command).
2. Optional, if you want a small code change too: have stage-demos.mjs try cargo build --release --bin gen-fixture (or cargo run --release --bin gen-fixture -- …) as a fallback before giving up, keeping the current warning if cargo is not available. Keep the script network-free.

Acceptance

• Fresh clone → cargo build --release --bin gen-fixture && cd viewer && npm ci && npm run stage:demos stages synthetic-mvp: ok, and the README says how.
• npm run typecheck and npm run build still pass (tools/check-local.sh --viewer runs both).

Scope notes

Docs (+ optional ~20-line script change); no corpus files needed; the demo is a license-free synthetic fixture.