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.