Account for UTXO base weight in anchor reserve checks

[tnull]

get_supportable_anchor_channels estimates how much each reserve UTXO can contribute after spending fees. Include the base input weight in that fee so UTXOs just below the public per-channel reserve are not counted as supporting another anchor channel.

Co-Authored-By: HAL 9000

This finding was discovered by Project Loupe

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No issues found.

The change in anchor_channel_reserves.rs:243-247 correctly includes BASE_INPUT_WEIGHT in the per-UTXO spend fee. Utxo.satisfaction_weight only covers the witness/scriptSig portion and excludes the base input weight (txid + vout + sequence), so adding BASE_INPUT_WEIGHT is necessary and matches the consistent pattern used everywhere else in the codebase (wallet_utils.rs, funding.rs, chan_utils.rs). The math checks out (BASE_INPUT_WEIGHT + p2wpkh satisfaction == P2WPKH_INPUT_WEIGHT), and the added test correctly exercises the off-by-one boundary that the fix addresses.

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[TheBlueMatt]

Backported to 0.2 in #4683.