fix(tracer): exec-check ptrace binary, not just file existence (P0-3)

[christophergeyer]

Summary

The ptrace tracer was treated as "available" anywhere a path-existence check returned True, even when the binary was a wrong-arch ELF that produced ENOEXEC at exec time. P0-3 reproduction on Mac (and reproducible on this aarch64 box by replacing the binary with non-ELF content):

$ roar tracer status
  ptrace:  /home/.../roar-tracer        # ← claimed available, no exec check
$ roar run python script.py
[Errno 8] Exec format error             # ← actually running it crashes

roar tracer check --backend ptrace and --backend auto were already exec-based (they go through the binary's --preflight --json and catch OSError(ENOEXEC)). The bug surfaced in three other places that all reduced ptrace's readiness to file-existence:

1. roar tracer status — printed ptrace: <path> with no readiness check, while ebpf/preload showed their ready state
2. Runtime auto-selection (_resolve_backend_candidate) — only called _find_ptrace_tracer, returned the candidate without exec-checking
3. _backend_ready_non_auto — same existence-only shortcut

Fix

Add ptrace_readiness(path) mirroring preload_readiness: exec the binary's --preflight --json once (cached via @cache), parse the JSON, surface either the binary's own failure summary or the OS error. Wire into all three call sites.

After-fix output (with broken ptrace binary)

$ roar tracer status
  ptrace:  /home/.../roar-tracer (ptrace tracer failed to exec: [Errno 8] Exec format error: '...')
  ebpf:    ...
  preload: ... (ready; ...)

$ roar tracer check --backend auto
Tracer check passed for 'auto': selected backend 'preload'   # ← falls through correctly

Test plan

• New tracer_backends unit tests: ptrace_is_ready happy path; ENOEXEC catch; binary-reports-its-own-failure path; auto-mode skips an unready ptrace and falls through
• New tracer_selection unit test: auto must skip ptrace when its readiness check fails (regression for the existence-only bug)
• Existing test_auto_skips_unready_ebpf_and_selects_ptrace updated to mock _ptrace_is_ready (the gate it didn't have before)
• pytest integration: 96 passed, 2 skipped, 0 failed
• pytest unit + happy_path: 1387 passed, 3 skipped, 0 failed
• ruff check / ruff format --check / mypy roar: clean
• Manual reproduction: replaced the ptrace binary with a non-ELF text file, confirmed all three commands now report the failure and --backend auto correctly falls through to preload

Stacks on #85.

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