setup-toolpath: digest-pinned prebuilt binary to replace the per-container compile

[bdelanghe]

Follow-up to #112. The SessionStart hook installs path via cargo install path-cli — registry-checksummed, but a real compile (heavy dep tree; minutes per fresh container, backgrounded so nothing waits). The ratchet is the bootstrap's own pattern: fetch a prebuilt binary from a pinned URL and refuse it unless it hashes to a digest recorded where it isn't fetched from (fetch_verified in .claude/README.md's canonical setup-script text).

Needs, in order:

• A reachable release host. github.com/empathic/toolpath releases 403 through the session proxy (out of repo scope), and pathbase.dev/install.sh pipes unverified bytes — fine for laptops, not for this hook. Options: add the release host to the environment allowlist, or mirror the binary somewhere already reachable (bounded.tools is on the allowlist).
• Capture the digest for a pinned version (now capturable live: the hook-built path 0.16.1 exists in-session to compare against a downloaded artifact).
• Teach setup-toolpath.sh to try the pinned binary first, keeping cargo install as the fallback and the same quiet-exit posture. Pin + digest bump together, like the bootstrap's PIN pair.

Not urgent: the compile path works and is verified (#112). This trades minutes of per-container background compile for a pinned fetch.