feat(bitcoin): Maestro data provider and first-class coin selection

[jinglescode]

Milestone: standard headless wallet on Bitcoin

Closes the remaining gap for the Bitcoin headless wallet milestone: the wallet, coin selection, and signTransfer already existed in src/, but no concrete data provider did. This PR adds the Maestro provider and promotes coin selection to a first-class, tested module.

A1 — UTXO querying via Maestro

• MaestroProvider (src/bitcoin/providers/maestro.ts) implements the existing IBitcoinProvider interface against Maestro's Esplora-compatible Bitcoin API: address info/UTXOs, per-tx unspent outputs (via /outspends), paginated tx history, tx status, sat/vB fee estimates (closest-target selection, testnet fallback), and raw-tx broadcast.
• Uses the platform fetch — zero new runtime dependencies. Shared plumbing in src/bitcoin/providers/common.ts with a typed ProviderHttpError.

A2 — Coin selection on queried UTXOs

• Extracted the largest-first selector into src/bitcoin/utils/coin-selection.ts and exported it (selectUtxosLargestFirst, DUST_THRESHOLD_P2WPKH). Behavior unchanged: BIP-141 vbyte fee model, 546-sat dust absorption. BitcoinHeadlessWallet.signTransfer now delegates to it.

A3 — Send transfer

• The existing signTransfer flow (query → select → PSBT → sign → broadcast) runs end-to-end on the Maestro provider; covered by the existing wallet regression tests.

Tests & docs

• 20 new provider tests (mocked HTTP) + 12 coin-selection tests; full suite green on this branch (231 tests, 19 suites), tsup CJS/ESM/DTS build passes.
• README "Bitcoin Headless Wallet" section and bitcoin-milestone.md mapping acceptance criteria to code.

🤖 Generated with Claude Code

[Temasar1]

✅ Bitcoin Provider Integration Demo

Tested using the new BlockstreamBitcoinProvider on Bitcoin Testnet.

> tsx bitcoin-run.ts

⚡  Bitcoin playground  [testnet]

────────────────────────────────────────────────────────────
Payment address : tb1qj6jl9g9c0j4qcwcxz4nn27c6rerz5xv30yprvs
Public key (hex): 02b628626f6dd36dabdd973a4e5d150c0b8b203a6f838ee2accaf9abe187fbaa11
────────────────────────────────────────────────────────────
Fetching balance…
  confirmed  : 0 sats
  unconfirmed: 5,000 sats
  total      : 5,000 sats
────────────────────────────────────────────────────────────
Fetching UTxOs…
  [0] 0b08ac6d03d87445060aed42451cec736c75fd1775511762e143e97b4670070a:1  5,000 sats  confirmed=false
────────────────────────────────────────────────────────────
Signing transfer (coin selection is automatic)…
  recipient : tb1qj6jl9g9c0j4qcwcxz4nn27c6rerz5xv30yprvs
  amount    : 4,000 sats
  ✓ Transaction submitted!
  txid: 007db8cdf25306f2d942c9367874a0e663f22a9b0ad80d8b62fed701df311ad4
────────────────────────────────────────────────────────────
Fetching tx history…
  [0] 0b08ac6d03d87445060aed42451cec736c75fd1775511762e143e97b4670070a  confirmed=false
  [1] 007db8cdf25306f2d942c9367874a0e663f22a9b0ad80d8b62fed701df311ad4  confirmed=false
────────────────────────────────────────────────────────────
✓ Done

https://mempool.space/testnet4/tx/007db8cdf25306f2d942c9367874a0e663f22a9b0ad80d8b62fed701df311ad4

Verified Functionality

• ✅ Generated a Bitcoin testnet wallet from a mnemonic
• ✅ Retrieved payment address and compressed public key
• ✅ Queried wallet balance
• ✅ Fetched available UTxOs using the @MeshJS/mesh-core provider package
• ✅ Automatically performed coin selection
• ✅ Built and signed a Bitcoin transaction
• ✅ Successfully submitted the transaction to the Bitcoin testnet
• ✅ Retrieved transaction history including the newly submitted transaction

This validates the end-to-end functionality of the new Bitcoin provider implementation, including read operations, transaction construction, signing, and broadcasting.