fix(phantom): connect Bitcoin via Wallet Standard when injected provider absent

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Problem

Connecting Phantom for Bitcoin throws SwapKitError: wallet_phantom_not_found. Other chains (Solana, Ethereum) work fine; Phantom is installed.

SwapKitError: wallet_phantom_not_found
    at C$1 (.../index-lI6q89jE.js:6662:27557)
    at Array.map (<anonymous>)
    ...

Root cause

Phantom has deprecated the injected window.phantom.bitcoin provider (docs) and newer builds expose Bitcoin only through the Wallet Standard registry. Solana and EVM are still injected (window.phantom.solana / window.phantom.ethereum), which is exactly why those chains keep working while BTC fails.

The Bitcoin connect path detected the provider solely via window.phantom.bitcoin?.isPhantom:

const provider = phantom?.bitcoin;
if (!provider?.isPhantom) throw new SwapKitError("wallet_phantom_not_found");

For users on newer Phantom builds window.phantom.bitcoin is absent → immediate throw. (Confirms hypothesis #1 of the investigation — the symptom asymmetry, BTC-only failure with Solana working on the same window.phantom, can only be explained by the injected BTC provider being gone.)

Fix

New getBitcoinAccess() resolves a Bitcoin signing surface in two steps:

1. Legacy injected provider first (window.phantom.bitcoin) — zero behaviour change for users where it still works.
2. Wallet Standard fallback — discovers Phantom via @wallet-standard/app getWallets() and uses the canonical Bitcoin Wallet Standard features:
   • bitcoin:connect → connect({ purposes: ["payment"] }) → { accounts } (uses the payment address, correct for swaps)
   • bitcoin:signTransaction → signTransaction({ psbt, inputsToSign: [{ account, signingIndexes }] }) → [{ signedPsbt }]

The getWalletMethods Bitcoin case now delegates to getBitcoinAccess; the toolbox/signer wiring is unchanged.

Tests

New packages/wallet-extensions/test/phantom-bitcoin.test.ts (deterministic, no network):

• ✅ falls back to Wallet Standard when injected provider absent (the original repro — fails on develop, passes here)
• ✅ legacy injected provider still works (regression)
• ✅ throws wallet_phantom_not_found when neither path available
• ✅ ignores a WS wallet missing required bitcoin features
• ✅ throws core_transaction_failed when WS signing returns no result

Full suite green (40 pass), typecheck + biome clean.

Preventing this class of bug

Other in-repo wallets still resolve BTC via deprecated injected providers and will break the same way if those wallets follow Phantom's migration (OKX, Bitget, OneKey, CTRL/XDEFI, KeepKey-bex, Vultisig). Worth a follow-up to standardise on Wallet Standard discovery for Bitcoin across extension wallets.

Note: the @near-js/* lines that may appear in bun.lock are pre-existing drift — a no-op bun install on develop already rewrites them; this PR's lock delta is limited to the new @wallet-standard/* entries.