qa: add fd-guard, the close() guard and regtest harness that found the double close

shim/ is an LD_PRELOAD guard around close/fclose/dup2/dup3:
- before each close it resolves /proc/self/fd/N;
- it reports, with a backtrace, a non-owner close of wallet.dat,
  database/log.* or blocks/*.dat, and every close that fails with EBADF
  (the second half of a double close);
- FDGUARD_ABORT=1 aborts on the first hit;
- run_tests.sh self-tests it (51 checks).

repro/ runs a shimmed dragonxd on isolated regtest. Each run gets its
own unprivileged network namespace (loopback only). The loads are:
- rpc: a hammer with impatient/RST/half-open clients, work-queue
  bursts and long-polls;
- p2p: TLS peers churned by disconnectnode, ban and kill, plus a raw
  TCP/TLS misbehaver;
- tor: a fake control port;
- mine: mining with blocknotify and autoshield.
tools/tally.py groups the reports by closer. tools/scan_bdb.py finds BDB
pages written into block files.

With the wolfSSL CloseSocket macro in effect:
- p2pmin (300 s) gives one EBADF double close per disconnect;
- the full load (1200 s) gives 930 EBADF closes, 186 non-owner closes
  (37 of wallet/log/blk files) and a 632 KB db.log with BDB0060 PANIC;
- RPC-only and Tor-only loads give none.
After the fix, every count is 0 and db.log stays empty. The README has
the commands and the numbers.

Paths come from the environment (GOLDEN, BIN, RUNS, SHIM, ASMAP and the
Sapling params). Run output goes to $RUNS, outside the tree.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

qa/fd-guard/repro/go.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# go.sh <name> <seconds>  -- create a private network namespace (loopback only, no route),
+# run run.sh inside it, then tear the namespace down. Env knobs pass through to run.sh.
+export REPRO=${REPRO:-$(cd "$(dirname "$0")" && pwd)}
+export RUNS=${RUNS:-$HOME/fd-guard-runs}
+OUT=$RUNS/$1; mkdir -p "$OUT"
+setsid unshare -rn sh -c "ip link set lo up && echo \$\$ > $OUT/ns.pid && exec sleep infinity" </dev/null >/dev/null 2>&1 &
+for _ in 1 2 3 4 5 6 7 8 9 10; do [ -s "$OUT/ns.pid" ] && break; sleep 0.3; done
+NSP=$(cat "$OUT/ns.pid")
+cp "$REPRO/run.sh" "$OUT/run.sh.snapshot"   # run a frozen copy: editing run.sh cannot disturb a live run
+nsenter -t "$NSP" -U -n --preserve-credentials bash "$OUT/run.sh.snapshot" "$@" > "$OUT/go.txt" 2>&1
+rc=$?
+kill "$NSP" 2>/dev/null
+echo "go.sh: run.sh rc=$rc, namespace holder $NSP killed" >> "$OUT/go.txt"