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: per-helper wire bench + shape-variety bench + multi-byte jit gates

Three additions so future perf regressions are visible at the right
granularity, not just averaged out by the single Person shape in
bench/bench.lua.

bench/wire_bench.lua (new). Microbenches every wire helper in
isolation — encode/decode for varint at 1/2/3/5 bytes, zigzag, fixed
widths, float/double, LEN, tag, skip_field, utf8 validator. Used when
tuning wire.lua to confirm a change moved the helper-level ns/op as
expected (e.g. encode_varint(150) 529 ns -> 68 ns from the 2-byte
fast path).  Run via `make bench-wire`. No baseline, no regression
gate — this is a manual inspection tool.

bench/shapes_bench.lua (new). Runs encode + decode against a handful
of fundamentally different Person / Event / Result shapes —
scalar-heavy, packed-int (100 and 1000 elements), nested-friends
(10 and 100), maps (scalar-valued and message-valued), oneof, and
WKT-heavy. Each shape dials one knob up so cost attribution stays
clean. Shows alloc B/op alongside throughput. Run via
`make bench-shapes`. Caught the packed-int / multi-byte-varint
opportunity that bench/bench.lua (all 1-byte varints, fixed
shape) doesn't surface.

bench/jit_trace.lua + 4 gates. Two new fixtures per mode that
specifically exercise encode_varint_slow's 2/3-byte Lua-number paths
and decode_string's multi-byte LEN fallback (200-byte name + packed
ints in [150..500000]). If a future change pushes encode_varint_slow
past LuaJIT's inline budget the gate fires instead of the regression
landing silently in shapes_bench. Total gate count: 19 -> 23.

Makefile gains `bench-wire` and `bench-shapes` phony targets.