pset: sign an issuing input over its own outpoint and denomination

[GracedEternalKingCabbageMan]

A caller that holds a whole transaction signs it by making a PSET from it (PartiallySignedTransaction::from_tx). For an input that issues an asset, that PSET extracted a different transaction from the one given, so the signature covered the wrong hash. The Arca server worked around both defects:

• The outpoint. Input::from_txin set bit 31 of the output index to mark the issuance. extract_tx and issuance_ids then read that index as the outpoint's, so the signed transaction spent output 2^31 + n, and the issued asset's id was computed over that outpoint.
• The denomination. Input::asset_issuance() wrote denomination 8 whatever the issuance said. Sequentia serializes the denomination after the inflation keys, so it is part of the signature hash.

The fix:

• The issuance is no longer flagged in the index; Elements Core's PSET does not flag it either.
• Input::previous_outpoint() masks off any flag a PSET carries there, as upstream rust-elements 0.27 does. The extracted transaction, the asset id and the kit's readers of an input's outpoint all use it.
• The denomination has no PSET field, so it is kept in a proprietary key (prefix sequentia, subtype 0x00, one byte). The key is written only when the denomination is not the default 8, so existing PSETs serialize exactly as before.

Each defect has a test that fails before this change:

• rust-elements an_issuance_input_keeps_its_outpoint: the outpoint came back with vout: 2147483648 instead of 0.
• an_issuance_input_keeps_its_denomination: the denomination came back as 8, not 0.
• lwk_signer an_issuing_input_is_signed_over_its_own_transaction: the kit's signature did not verify against the transaction's own sighash ("signature failed verification").

On regtest (node v24.7.12, elementsregtest, -par=1), a P2WPKH coin issued an asset and was signed through the kit's PSET signer:

• Before the fix, it was refused in the mempool (mempool-script-verify-flag-failed (Signature must be zero for failed CHECK(MULTI)SIG operation)). Forced into a block with generateblock, it was refused again (TestBlockValidity failed: mempool-script-verify-flag-failed (Script evaluated without error but finished with a false/empty top stack element)). This held at denominations 8 and 2.
• After the fix, coins at vout 1 with denominations 8, 2 and 0 were accepted and confirmed, and the node reports each issuance's denomination as set.
• A PSET whose denomination key is removed is still refused, with the same two errors.

Suites: rust-elements 91 tests without sequentia and 86 with it, plus 14 doc-tests; lwk_signer 26 (27 with ark); lwk_common 40; lwk_wollet ark:: 22.