fix: create socket dir before spawning session daemon

[ccakes]

Problem

Starting a session fails on a clean /tmp with:

error: spawning session daemon: open /tmp/bench-<uid>/bench-<hash>.log: no such file or directory

The launcher (ensureSession) writes the detached daemon's .log into the per-user socket directory /tmp/bench-<uid>/ before spawning the daemon — but that directory is only created later by the daemon itself (Server.Start → EnsureSocketDir) when it binds its socket. macOS reaps /tmp entries after ~3 days, so a directory from a prior session is gone by the next launch and the log open (os.OpenFile with O_CREATE, which does not create parents) fails with ENOENT.

Regression from the detachable-sessions feature in 0.6.9.

Fix

ensureSession now calls api.EnsureSocketDir(sockPath) right before opening the log file, reusing the existing 0700 + symlink hardening the daemon already applies. The call is idempotent with the daemon's own, so the daemon path is unchanged.

ensurePrivateSocketDir is exported as EnsureSocketDir so the cli package can call it.

Tests

internal/api/socketdir_test.go (drives os.TempDir() via TMPDIR):

• creates the private dir when missing, asserts 0700, and is idempotent
• no-op for socket paths outside the private dir
• rejects a symlinked private dir

Verification

Built a binary from the pre-fix cli.go and reproduced the exact error on a clean slate; the fixed binary starts cleanly and creates the dir (drwx------) containing both the .log (0600) and .sock. gofmt -l, go vet ./..., golangci-lint run, and go test ./... all clean.

Note for reviewers

EnsureSocketDir is intentionally a no-op for custom --socket/BENCH_SOCKET paths outside the private dir, so a custom socket in a nonexistent directory would still hit the same log-open failure. Left as-is: that path is user-chosen and net.Listen would surface a clear error there anyway. Happy to widen it if you'd prefer.

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