[corpetty]

Revives #289 by rebasing it onto current main. The only conflict was Settings.qml, where main had reorganized the identity section: the Bitcoin key row was already there, one level deeper. I kept main's layout and inserted only this PR's new block (the payer-address rows) after the key row, re-indented to match. Also carries a pebbles note (exo-80b) for #321.

Re-tested on main (2026-10-17):

• suite 263/263, including the new btc_payer_address_test with BIP-173 vectors;
• both .lgx packages build;
• ui-lint has only the 2 baseline-new findings already on main;
• on display, in Basecamp 0.3.2 (release AppImage, this build installed): Settings → Advanced shows the rows under Your Bitcoin key, each with Copy: mainnet bc1q…, testnet · testnet4 · signet tb1q…, regtest bcrt1q….

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Original description:

Item 12 of exo-408. A Bitcoin split's debtor pays from wpkh(<their muster key>), with coins from their own node, but no screen showed that address, only the key.

• payerAddresses (bitcoin/script) gives wpkh of the key on every network, pinned to BIP-173's own vectors.
• identity() and settings() return it as btcPayers: [{network, chain, address}].
• Settings lists one address per bech32 prefix, naming the networks that share it (testnet, testnet4 and signet all use tb1…), each with Copy.
• identity()'s contract description, stale since the two-identity model, now names what it returns.

Tests:

• btc_payer_address_test: the BIP-173 mainnet and testnet vectors, and one row per network with its own chain and prefix.
• The Bitcoin tests pass (8/8).
• nim check of the hosted module is clean, and ui-lint has no findings.
• A new gallery fixture, settings-btc-payer, was recorded from scripts/split-self-test.sh on a runner built with this change and asserts the rows are visible.

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