docs(design): SEC-1.r temp_guard sandbox follow-up (closes 5 GAPs from #4710)

[oferchen]

Summary

• Opens SEC-1.r tracking doc for the 5 daemon-reachable GAP sites in temp_guard / temp_cleanup surfaced by the SEC-1 path-syscall coverage audit (docs(audit): SEC-1 path-syscall coverage audit -- post-chain GAPs survey #4710, rows Add negotiation prologue detection helper #17-Add byte-based legacy daemon message parser #21).
• Documents the prerequisite openat_via_sandbox_or_fallback helper (also closes ~11 other audit GAPs - rows Add helpers for supported protocol versions #9-Refine legacy greeting parser whitespace handling #13 inplace/basis opens).
• Recommends a 4-step approach: ship helper standalone, extend TempFileGuard with a sandbox anchor, thread sandbox through open_tmpfile, wire the two callers (disk_commit/process.rs:248, transfer_ops/response.rs:91). Optional step 5 adds read_dir_via_sandbox_or_fallback to close rows Expose NegotiationPrologueDetector decision state #20-Add byte-based legacy daemon message parser #21.
• Estimated ~200-300 LoC. Re-open triggers cover any TOCTOU finding on the temp path, helper landing, or SEC-1.j cross-thread closure landing first.

Test plan

• Doc-only change; verified git status shows the new design file only.
• No code touched; CI fmt/clippy/nextest checks should pass unchanged.