bench: apply mcode arena hardening across all bench scripts (3qu)

Added jit.opt.start('sizemcode=64', 'maxmcode=4096') to all 9 bench
scripts. Same fix 3o2 landed in bench/jit_trace.lua, propagated to the
rest. Without it, the macOS arm64 mcode allocator intermittently fails
to find an executable page in the signed-32-bit offset window and the
bench reports interpreter-only throughput with no diagnostic — visible
as silent regressions. The risk is higher in scripts with larger codegen
footprints than the trace gate.

Files touched: bench.lua, lazy_bench.lua, profile.lua, shapes_bench.lua,
starwing_bench.lua, wire_bench.lua, alloc_probe.lua, map_bench.lua,
packed_bench.lua. All 7 non-interactive scripts run rc=0; profile and
starwing loadfile-check clean.

Also refreshed bench/baseline.json per the issue's step 2. Surprise win:
full-mode decode allocations dropped 0.5-50% as a side-effect of auj/ozn
that was only visible after the snapshot. Most notable: proto2_basic
BenchPayload mid decode 2.313 -> 1.156 KB/op (-50%), Person 1KB decode
0.977 -> 0.953 KB/op. Runtime-mode unchanged — confirms the alloc
reduction is full-mode codegen specific.

Steps 3-4 (refresh COMPARISON.md throughput tables, verify the variance
band shrinks below the documented 5-10% drift) are deferred to a
work.lab.local run — laptop variance is 50%+ on identical state, can't
trust throughput A/Bs locally.

bench: alloc decomposition probe

Adds bench/alloc_probe.lua, a surgical probe that strips the
encode path apart and measures KB-delta per primitive operation.
Lets us attribute the ~136 B/op encode floor (captured in the
perf-analysis-2026-05-18 memo) to a specific source: result
table, intermediate varint strings, output string, etc.

Runs with GC stopped so allocations accumulate; divides by N to
get bytes-per-op. Uses unique input bytes per iteration where
appropriate to eliminate string-interning noise.

Complements bench/profile.lua (where time goes) with where
*memory* goes — the two together informed which bd perf items
were worth landing (the bytes-saved had to translate to a real
allocation rate change, not just a hot-line attribution).