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.

mise.toml

@@ -16,10 +16,11 @@
 rust = { version = "stable", components = "clippy,rustfmt,rust-src,llvm-tools-preview", targets = "thumbv7em-none-eabihf,thumbv6m-none-eabi,thumbv8m.main-none-eabihf,riscv32imac-unknown-none-elf" }
 
 # ── Cargo subcommands the justfile drives ───────────────────────────
-"cargo:cargo-nextest"  = "latest"   # `just test`, `just dev`
-"cargo:cargo-llvm-cov" = "latest"   # `just coverage`
-"cargo:cargo-mutants"  = "latest"   # `just mutants`
-"cargo:cargo-audit"    = "latest"   # `just audit`
+"cargo:cargo-nextest"   = "latest"   # `just test`, `just dev`
+"cargo:cargo-llvm-cov"  = "latest"   # `just coverage`
+"cargo:cargo-mutants"   = "latest"   # `just mutants`
+"cargo:cargo-audit"     = "latest"   # `just audit`
+"cargo:gungraun-runner" = "latest"   # `cargo bench --bench dispatch` (valgrind harness)
 
 # ── Build orchestration ─────────────────────────────────────────────
 just = "latest"