bench: commit the harnesses the frank-medium numbers came from

Every measurement behind the frank-medium runbook was taken with scripts that
lived in /tmp on two hosts — reproducible only until the next reboot, and
invisible to anyone reading the repository. This moves the useful four into
scripts/, where this repo already keeps its bench harnesses:

  bench-vllm-corpus.py    code-shaped corpus from the repo's own sources, because
                          synthetic random prompts misreport draft acceptance
                          badly (2.9-3.6 tok/update against ~5.8 on code)
  bench-vllm-serve.sh     concurrency sweep; encodes the two traps that produced
                          silent garbage — the tokenizer argument, without which
                          the client dies resolving "frank-medium" on HF and
                          reports no numbers at all, and the --ignore-eos
                          semantics that must not be compared across
  bench-turn-latency.py   per-turn TTFT for an agent-loop shape; the instrument
                          that decided the routing policy, and the one that shows
                          cold-vs-warm prefill directly
  bench-llm-soak.py       sustained multi-family load; what found the capacity
                          wall (three 30k-token families, 75% cold re-prefills,
                          zero failures)

Each header carries its traps and the interpretation; the runbook's Benchmarks
section now points at them instead of describing methodology with no tooling.

Also adds the missing half of the script seam: scripts/*.py are compiled, not
just scripts/*.sh syntax-checked — a harness with a syntax error could otherwise
ship unnoticed until someone needed it during an incident.

Both python harnesses were smoke-tested against the live fleet before commit:
the corpus builder wrote 15 prompts, and the turn harness measured 12.5 s cold
against 1.4 s warm on the same prompt — the cache behaviour the runbook claims.

tests/scripts_syntax.bats

@@ -29,3 +29,16 @@ load helpers
         skip "shellcheck not installed locally"
     fi
 }
+
+@test "every python script in scripts/ compiles" {
+    # The shell half of this seam covered scripts/*.sh; the python half was only
+    # covered where a test happened to reference a specific file, so a syntax
+    # error in a committed harness could ship unnoticed until someone ran it
+    # during an incident.
+    local any=0
+    while IFS= read -r p; do
+        any=1
+        python3 -m py_compile "$p" || { echo "syntax error in $p"; return 1; }
+    done < <(find "$REPO_ROOT/scripts" -name '*.py' -type f 2>/dev/null | sort)
+    [ "$any" -eq 1 ] || skip "no python scripts yet"
+}