feat(fuzz): cargo-fuzz workspace, 3 wire-surface targets; bump alkcall 0.8.1

- Bump alkcall 0.8.0 -> 0.8.1 (duplicate adopt/open channel-state
  destruction fix from alkcall's fuzz campaign; AGENTS.md stale-pin fix).
- Adopt fuzzing per docs/plans/fuzzing.md (mirrors alkcall's
  docs/research/fuzzing.md as-built layout): fuzz/ workspace with own
  [workspace] table, nightly pinned for the subtree only, invariant
  logic in a stable-toolchain shared crate.
- Targets: chunk_frame (5-byte chunk codec), negotiation_frame
  (negotiation framing + NegotiateRequest + error_response_bytes +
  the cross-codec peek-disambiguation seam), control_json
  (ControlMessage JSON + signal_from_name). src/local/ out of scope
  (not wire-attacker-shaped).
- 328 committed seeds (deterministic generator); grown corpora
  gitignored. Corpus replay on stable is the standing fuzz gate
  (cargo test --manifest-path fuzz/shared/Cargo.toml).
- Detached-runner rule (fuzz/run-detached.sh): campaigns never run in
  the foreground of an agent session - OOM in a target must cost the
  fuzzer, never the session host.
- Root Cargo.toml gained [workspace] members/exclude (MSRV vs fuzz
  nightly dev-deps footgun) and fuzz/ in the publish exclude.

Verification: cargo test (113), cargo test --all-features (156),
clippy stable + wasm32-unknown-unknown, fmt, doc, publish dry-run,
corpus replay 328 seeds - all green. 10-min detached campaigns on all
three targets: 0 crashes/hangs/OOMs/leaks (chunk_frame cov-saturated
at 710 edges; negotiation_frame 3.69M execs cov 3157; control_json
8.6M execs cov 2035). Corpus replay caught three harness-model
mismatches pre-campaign (payload-slice shape; the negotiation framing
layer is length-prefix-only - Json unreachable from read_frame;
zero-length frames admitted by the reader, rejected by the adapter's
parse). Campaigns recorded in docs/plans/fuzzing.md section 8.

fuzz/run-detached.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+# Detached fuzzing runner for agent sessions: the fuzz campaign never
+# runs as a foreground child of the session (OOM in a target must not
+# take down the agent host), and survives the session ending.
+#
+# Usage: fuzz/run-detached.sh <target> [extra libfuzzer args...]
+# (works from the repo root or from fuzz/; CWD-independent)
+# FUZZ_RUNTIME_SECS overrides the per-campaign budget (default 600 s).
+#
+# Poll instead of waiting:
+#   tail -n 50 fuzz/artifacts/<target>-*.log
+#   ls fuzz/artifacts/<target>/   (crash-* / oom-* / timeout-* files)
+#   pgrep -f "cargo fuzz run <target>"
+set -euo pipefail
+target="${1:?usage: run-detached.sh <target> [extra libfuzzer args...]}"
+shift
+
+root="$(git rev-parse --show-toplevel)"
+fuzz_dir="$root/fuzz"
+mkdir -p "$fuzz_dir/artifacts"
+runtime="${FUZZ_RUNTIME_SECS:-600}"
+log="$fuzz_dir/artifacts/${target}-$(date -u +%Y%m%d-%H%M%S).log"
+
+cd "$fuzz_dir"
+setsid nohup cargo fuzz run "$target" -- \
+  -fork=1 -rss_limit_mb=2048 -malloc_limit_mb=2048 -timeout=25 \
+  -max_total_time="$runtime" "$@" \
+  >"$log" 2>&1 < /dev/null &
+echo "pid=$! log=$log"