chore(bench): migrate dispatch harness from iai-callgrind to gungraun

iai-callgrind is unmaintained; gungraun is the active fork. API is
drop-in (use iai_callgrind → use gungraun, same library_benchmark /
library_benchmark_group / main! macros), so the dispatch.rs body only
needed the import rename.

Also:
- mise.toml: add cargo:gungraun-runner dep so the valgrind harness
  binary tracks the lib version automatically.
- baseline.txt: rebaselined under gungraun_initial. Absolute numbers
  shifted up because gungraun's bench wrapper instruments more than
  iai-callgrind 0.16's did; comparable benches (terminate_event 69→68,
  h8_cross_tree 93→97) confirm HSM dispatch work itself is unchanged.
  Old iai-callgrind named baselines kept as historical context only.

docs/003. latency-analysis.md

@@ -484,7 +484,7 @@ Cost remaining, in priority order:
    users with all-sync actions skip this entirely. **User-facing API
    change** — out of scope for the current optimization pass.
 2. **Bench-wrapper noise floor** — at the smallest dispatch's ~38–80
-   instructions, the iai-callgrind wrapper (`black_box`, `send`,
+   instructions, the gungraun wrapper (`black_box`, `send`,
    instrumented call-edge crossing) accounts for ~45 instructions.
    Sub-bench-wrapper improvements are below the measurement floor.
 3. **Memory cost of `__PATH_DATA` / `__PATH_RANGE` / `__DISPATCH` /