Fix plain channel interoperability with stock LDK

[bitwalt]

Fixes #35.

Makes the two RGB wire/transaction extensions conditional, so plain channels are byte-identical to upstream LDK while RGB channels are unaffected.

Commitment transaction fee

commitment_tx_base_weight goes back to upstream, and the OP_RETURN weight is added only when the channel is colored. SpecTxBuilder carries that flag, taken from the rgb_asset already on FundingScope, so every fee calculation on a channel agrees with the transaction actually built for it.

The constant and the colored fee helper live in rgb_utils, next to the rest of the RGB logic, so chan_utils and tx_builder only wire it in. The weight is documented where it is defined: 8-byte value, 1-byte script length and a 34-byte OP_RETURN OP_PUSHBYTES_32 <commitment> script, so 43 vbytes, 172 weight units.

Two details worth pointing out for review:

• get_next_commitment_stats now uses the same commit_tx_fee_sat as build_commitment_transaction. It previously called the free function directly, which was harmless while every path unconditionally included the OP_RETURN weight, but would have left colored channels predicting a fee 172 weight units below the one baked into their own commitment transaction.
• The colored helper keeps the fee in a single division rather than adding the OP_RETURN cost separately, so rounding matches the uncolored path exactly.

update_add_htlc

rgb_payment becomes an optional odd TLV instead of a mandatory suffix, so peers without RGB support skip it.

Open question: we chose an odd type outside the range the BOLTs may assign, following the convention already used here for skimmed_fee_msat (65537) and hold_htlc (75537), where the TODO above hold_htlc spells out the reasoning. If RGB has a preferred allocation, or you would rather this be specified somewhere first, say which type and we will match it — it is a one-line change.

Validation

Verified at the node level rather than here, since that is where a stock peer can actually be driven: RGB-Tools/rgb-lightning-node#149 adds a stock ldk-node fixture and exercises the full lifecycle in both opener directions — channel_ready, a payment each way, balances moving as expected, cooperative close settling on-chain — plus a negative test asserting an RGB channel to a stock peer never becomes ready. All three pass with this commit and fail without it.

Compatibility

Breaking on the wire for deployed nodes: plain channels between a fixed and an unfixed peer derive different commitment transactions and force-close on the next commitment_signed, and RGB channels cannot parse each other's HTLCs across the encoding change. Persisted channel state is unaffected, so restarting an upgraded node is safe, but a release carrying this wants a coordinated upgrade.