STM BLS verification WORKING against live preprod (milestones A, B, partial C)

Key findings from upstream:
- Mithril's BLS msg is NOT signed_message alone — it's
  msgp = signed_message_ascii_bytes || mt_commitment_root_32_bytes
- Mithril uses EMPTY DST for hash-to-G1 (not the IETF BLS suite string)
- Aggregation is NOT plain summation — it's MuSig-style weighted:
  t_i = Blake2b-128(Blake2b-128(sigs_concat) || be_u64(i))
  aggr_sig = Σ t_i · sig_i      (in G1)
  aggr_vk  = Σ t_i · vk_i       (in G2)
  This blocks rogue-key attacks.

Shipped:
- internal/stm/types.go: MultiSig + AVK decoders (hex-of-ASCII-JSON wrapping,
  polymorphic tuple JSON handling via ByteArray + custom UnmarshalJSON)
- internal/stm/bls.go: BlsVerify (pairing check with gnark-crypto)
- internal/stm/aggregate.go: MuSig-style AggregateBLS + BlsAggregateVerify
- synthetic test + live test (build tag 'live') both green

Live preprod head cert (epoch 284, cert 175051cf…):
- 2 signers, 11 total lottery wins
- aggregate verify: PASS ✓
- single-signer verify: PASS ✓

Next: lottery threshold check, Merkle batch-proof verification, glue into
top-level Verify(msg, multi_sig, avk, params) + wire to 'verify' subcommand.

go.sum

@@ -1,2 +1,12 @@
+github.com/bits-and-blooms/bitset v1.24.4 h1:95H15Og1clikBrKr/DuzMXkQzECs1M6hhoGXLwLQOZE=
+github.com/bits-and-blooms/bitset v1.24.4/go.mod h1:7hO7Gc7Pp1vODcmWvKMRA9BNmbv6a/7QIWpPxHddWR8=
+github.com/consensys/gnark-crypto v0.20.1 h1:PXDUBvk8AzhvWowHLWBEAfUQcV1/aZgWIqD6eMpXmDg=
+github.com/consensys/gnark-crypto v0.20.1/go.mod h1:RBWrSgy+IDbGR69RRV313th3M/aZU1ubk2om+qHuTSc=
 github.com/klauspost/compress v1.18.5 h1:/h1gH5Ce+VWNLSWqPzOVn6XBO+vJbCNGvjoaGBFW2IE=
 github.com/klauspost/compress v1.18.5/go.mod h1:cwPg85FWrGar70rWktvGQj8/hthj3wpl0PGDogxkrSQ=
+golang.org/x/crypto v0.50.0 h1:zO47/JPrL6vsNkINmLoo/PH1gcxpls50DNogFvB5ZGI=
+golang.org/x/crypto v0.50.0/go.mod h1:3muZ7vA7PBCE6xgPX7nkzzjiUq87kRItoJQM1Yo8S+Q=
+golang.org/x/sys v0.41.0 h1:Ivj+2Cp/ylzLiEU89QhWblYnOE9zerudt9Ftecq2C6k=
+golang.org/x/sys v0.41.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
+golang.org/x/sys v0.43.0 h1:Rlag2XtaFTxp19wS8MXlJwTvoh8ArU6ezoyFsMyCTNI=
+golang.org/x/sys v0.43.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=