fix(config): swap dead BTCB Chainlink address for live AggregatorV3 (closes #109)

[obchain]

Summary

• Swap dead BTCB Chainlink feed 0x264990fbd0…1f0 (no bytecode on BSC mainnet) for the live AggregatorV3 0x8ECF7dE377F788A813F5215668E282556b35f300 discovered via the Venus oracle path.
• Apply the same swap in config/fork.toml (forks mainnet block, hits same dead address).
• Add a comment block above [chainlink.bnb] documenting the discovery path.

Closes #109 (re-fixed — the previous fix shipped a still-wrong address).

Why

cast code 0x264990fbd0A4796A3E3d8E37022BdAf1A5a4C1f0 --rpc-url https://bsc.drpc.org → 0x. EOA, no contract. Every BTCB price call has been silently failing with ABI decoding failed: buffer overrun while deserializing since PR #35.

Live charon log against the dead address:

WARN charon_scanner::oracle: chainlink refresh failed
  symbol=BTCB
  err=feed 'BTCB' (0x264990FBd0a4796a3E3d8E37022BdAf1A5a4C1f0): decimals() failed
       ABI decoding failed: buffer overrun while deserializing

Verification

$ cast code 0x8ECF7dE377F788A813F5215668E282556b35f300 --rpc-url https://bsc.drpc.org | head -c 30
0x6080604052348015610010576000

$ cast call 0x8ECF7dE377F788A813F5215668E282556b35f300 'decimals()(uint8)' --rpc-url https://bsc.drpc.org
18

$ cast call 0x8ECF7dE377F788A813F5215668E282556b35f300 'description()(string)' --rpc-url https://bsc.drpc.org
"BTC / USD"

$ cast call 0x8ECF7dE377F788A813F5215668E282556b35f300 'latestRoundData()(uint80,int256,uint256,uint256,uint80)' --rpc-url https://bsc.drpc.org
36893488147419392801
78090021016201850000000   # = $78,090
1777191159
1777191162
36893488147419392801

$ cast call 0x8ECF7dE377F788A813F5215668E282556b35f300 'owner()(address)' --rpc-url https://bsc.drpc.org
0xcb6754D5e11C4B70601D0F2068eA62089325979b   # same as BNB/USD owner — Chainlink-ops multisig

Decimal difference (no Rust change needed)

The new aggregator is 18-decimal, not 8 like the other feeds. PriceCache reads decimals() per feed at startup (crates/charon-scanner/src/oracle.rs:59) and CachedPrice::scaled_to rescales to any target — the difference is invisible to consumers.

Reviewer findings (blockchain-code-reviewer)

All applied:

• Confirmed live address is Chainlink-ops-controlled, not a Venus wrapper (owner-match check).
• Found config/fork.toml carries the same dead address — patched.
• Added discovery-path comment so future reviewers don't have to redo the walk.
• Verified no Rust source / test fixture pins the old address (grep -rn 264990fb returned only the two TOML hits).
• Flagged for separate issue: the other four [chainlink.bnb] entries deserve the same cast code sanity check.

Test plan

• cast code confirms bytecode on the new address (both dRPC and bsc-dataseed.binance.org)
• decimals(), description(), latestRoundData() all succeed
• owner() matches BNB/USD canonical feed owner
• No Rust changes required — PriceCache is decimal-agnostic
• CI gates (cargo / forge / docker)