fix: pin async-substrate-interface==1.6.4 + bump bt-decode to 0.8.0

[elliot-bitcast]

Problem

async-substrate-interface>=1.4.2 was an open-ended floor pin. pip resolves this to 2.2.0 (latest), which has two breaking changes that crash import bittensor:

1. scalecodec/cyscale conflict — 2.x adds a runtime conflict check that raises RuntimeError on import
2. ScaleObj removed from async_substrate_interface.types — bittensor 9.12.2 imports it directly → ImportError

This was reported by a miner/validator hitting the error on fresh install.

Fix

• Pin async-substrate-interface==1.6.4 (latest compatible 1.x, same as bitcast-x)
• Bump bt-decode==0.6.0 → 0.8.0 (required by async-substrate-interface 1.6.4)

Verification

Reproduced the crash with 2.2.0, then verified the fix:

pip install -r requirements.txt  ✅ clean install
import bittensor                   ✅ (v9.12.2)
from async_substrate_interface.types import ScaleObj  ✅
from async_substrate_interface import SubstrateInterface  ✅
import bt_decode                   ✅

Context

Same fix already applied to bitcast-x in commit 3cb60e2 (4 May 2026). This PR brings the main neuron repo in line.