Fix example test failures on Windows hosts

[NawaMan]

Summary

A full examples/workspaces/run-example-tests.sh run on Windows (Git Bash + Docker Desktop) had 14 of 69 examples failing. This fixes the causes that belong to the tests and to the dind setup.

Example tests (8286e644)

• Stale wrapper path (10 examples): 17 scripts called ../../../codingbooth relative to the cwd; the wrapper is now booth. They now resolve the wrapper from their own location (preferring booth, falling back to codingbooth) and fail with a clear message if neither exists.
• Emoji checks (all-java, java, go): Git Bash's grep 3.0 cannot match 4-byte UTF-8 emoji under a UTF-8 locale (-F doesn't help). Those checks now run LC_ALL=C grep to match bytes.
• js-example: test002 checked the servers once after a fixed 2s sleep; the tsx-compiled API server often isn't up yet. It now polls for up to 60s.
• zig-example: Zig's rename into a bind-mounted .zig-cache fails with AccessDenied; the Justfile sets ZIG_LOCAL_CACHE_DIR to /tmp/zig-cache (overridable).

dind: host-only credential helpers (ff71ccdc)

Docker Desktop writes "credsStore": "desktop" into ~/.docker/config.json. The docker-config extension seeds that into the booth, where docker-credential-desktop doesn't exist, so every docker pull fails — floci and appwrite autostart never come up.

dind--setup.sh now installs /usr/share/startup.d/40-cb-dind--startup.sh, which removes credsStore/credHelpers entries whose docker-credential-* binary isn't on PATH, keeping auths and everything else. It is numbered before the autostart segments and never fails the boot. New unit test: tests/setups/test--dind-docker-config-helpers.sh.

Testing

• The 10 path-affected examples plus all-java, java, js, go and zig pass individually on Windows.
• test--dind-docker-config-helpers.sh: 6/6 cases pass inside the base image (it prints SKIP on hosts without jq).
• In a floci-example booth, running the hook removed desktop, floci/floci:latest pulled, and the S3 round-trip succeeded.
• Not yet verified: floci-example / appwrite-example through the test runner. They need a base image containing this change, and a locally rebuilt base image is currently ignored — see below.

Known follow-up

The Boothfile compiler emits # syntax=docker/dockerfile:1.7 (cli/src/pkg/boothfile/compiler.go). With that directive the external frontend resolves FROM against the registry, ignoring --pull=false and any local image, so locally rebuilt base images are never used by booth builds. Not addressed here.

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