Run bun test files in parallel (fixes #261)

[theogravity]

What

Turns on bun test --parallel for every package and removes the two hazards that only survived serial execution (measured facts, probed on 1.4.0 and 1.4.2):

• tmux sweep: a preload afterAll fires once per run serially but after EVERY file under --parallel. The run-wide tmux kill-all moved from test-preload.ts to scripts/test-run.sh, which outlives every worker. (The leak net itself predates this PR; see docs/testing-notes.md.)
• implicit DB boot: five server suites reached the shared @/db singleton without migrating it, riding file order in the one-process serial run. They now call ensureMigratedTestDb() (+ prepareLocalPlugins() where the restart gate needs it) in beforeAll. Idempotent, serial stays green.

Then the flag: --parallel=12 for server/api, server/web, node/agent; --parallel=4 everywhere else; in the test scripts (bun ignores a parallel key in bunfig.toml, measured; bun.com/docs/test lists it as CLI-only).

Numbers (48-core dev machine, 51 turbo tasks)

package	serial	parallel
apps/server/api	115.6 s	40.1 s
apps/server/web	58.8 s	11.3 s
apps/node/agent	13.6 s	6.0 s
packages/pane-runtime	18.7 s	14.0 s

Verification

• Three consecutive full bun run test (51/51 tasks, 0 failed tests) on bun 1.4.2, plus a --parallel=1 serial gate on the server package and test:scripts; verify-types, lint:check, lint:prose clean.
• A local flake from the bake (@internal/desktop-server meta-test pinning the test-script string) is fixed in 1d2b919.
• Dev machine bun upgraded 1.4.0 → 1.4.2 to match CI's builder image; no lockfile churn.

Design + plan: docs/superpowers/specs/2026-09-29-parallel-tests-design.md, docs/superpowers/plans/2026-09-29-parallel-tests.md. CI --shard split of test-server-node follows as a stacked PR.

No changeset: test infra only, no shipped code changes.