fix(builder): build the dev host where it's served, and repair the stale obc-pack paths

[timohueser]

Two independent path bugs in builder/, found one after the other. Both are the
silent kind: nothing throws, so nothing noticed.

1. The dev host's bundle was tracked, and served from nowhere

builder/static/dist/ was a Vite build artifact committed by accident in
c070d62. Vite's dev outDir was ../static/dist relative to builder/app,
resolving to builder/static/dist — but server.py, tools/justfile, the
README and .gitignore all name builder/server/static/dist. Only the ignore
rule was expected to cover it, so 640 KB of bundle landed in the tree, and
a6b0c35 later swept 36 more files of it into an unrelated chore commit.

The same mismatch meant the FastAPI dev host was broken: it mounts a
directory that never existed, so python -m builder.server served its
"Frontend not built yet" page even immediately after npm run build.

Fixed by pointing the outDir at the path the other four references already
agreed on, then untracking the misplaced copy. The .gitignore line was
already correct and needed no change.

2. /api/schema's fallback could never fire

SCHEMA_FILE still named firmware/obc-pack/…, a path that stopped existing
when the crate moved to host/. The binary branch kept working, os.path.exists
quietly went False, and the repo-file fallback stopped firing — so an unbuilt
packer took the 503 instead of degrading to a usable schema, and the editor lost
the capability information it derives from that envelope.

ci.yml builds obc-pack immediately before pytest, so the existing /api/schema
test always took the binary branch and could not have caught this. The Rust side
pins the file's contents through include_str! (a move there is a compile
error) and the frontend imports it by relative path — Python's string-joined path
was the only one that could rot in silence. builder/tests/test_schema_source.py
now pins it, along with the fallback branch actually answering and both sources
producing the same envelope shape.

Also fixed: the build hints in server.py and jobs.py said in firmware/, and
both schema staleness tests told you to regenerate into obc-pack/schema/… —
followed from the repo root that creates a stray top-level obc-pack/ directory
and leaves the test still failing. firmware/obc-pack now appears nowhere.

Verification

• npm run build writes builder/server/static/dist (same asset hashes the
  committed bundle had), the server serves it at 200 with no console errors, and
  git status stays clean after a build.
• With the directory absent, / still returns the existing 503 build-me page
  rather than a stack trace.
• builder/tests/: 10 passed with obc-pack built. The three new path tests were
  checked against the pre-fix code and all three fail there.
• cargo test -p obc-pack: 199 passed. cargo fmt --all clean.

Docs

README.md's three-hosts table pointed at frontend/dist/<host>/, the app
directory's pre-refactor name — corrected to builder/app/dist/ in its own
docs: commit. Nothing in docs/content/ describes these paths or the schema
fallback, so the docs site needs no update.

🤖 Generated with Claude Code