fix: persist WalletConnect session, negative-weight detection, Freighter/XBull error handling (#771–774)

[ushpraise]

What & Why

This PR resolves four Wave issues in a single change-set. Each fix is scoped to the file named in the issue brief.

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#774 — Persist WalletConnect session across page reloads

Problem: WalletConnectAdapter created a session but never saved it, so every page reload forced users to re-scan a QR code.

Fix:

• Constructor now writes { topic, relayUrl, chainId, address, expiry } to localStorage under the key stellarsplit:wc:session.
• New static factory WalletConnectAdapter.restore(client) reads the stored session, validates the WalletConnect expiry timestamp, and returns a ready adapter — no new QR scan needed.
• disconnect() removes the persisted entry.
• Accepts optional relayUrl and expiry constructor options.

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#773 — Detect negative-weight edges in PaymentGraphChecker

Problem: Negative-weight edges could cause unbounded fund extraction when the graph is traversed greedily; the checker did not detect them.

Fix:

• New checkGraph(graph: PaymentGraph): GraphValidationResult method on PaymentGraphChecker.
• Returns { valid: false, reason } on the first offending edge, naming source → target and the weight in the message.
• Zero-weight (pass-through) edges are explicitly allowed.
• Exports PaymentGraph, PaymentGraphEdge, and GraphValidationResult types.

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#772 — Handle Freighter not-installed error

Problem: FreighterAdapter propagated raw TypeError when the Freighter browser extension was absent.

Fix:

• New exported FreighterNotInstalledError (extends Error, proper instanceof) with the install URL (https://www.freighter.app) in its message.
• Private assertInstalled() helper called at the top of every public method (connect, sign, getAddress, signTransaction).
• Existing connection flow is unchanged when the extension is present.

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#771 — Check XBull extension version before connection

Problem: XBullAdapter connected without verifying the extension version, causing cryptic errors with older builds.

Fix:

• Exports MIN_XBULL_VERSION = '2.0.0' — the first version supporting the current API surface.
• New exported ExtensionVersionError with installedVersion / requiredVersion properties.
• connect() reads window.xBullSDK.version and throws ExtensionVersionError when below the minimum.
• If window.xBullSDK is absent (older extension that doesn't expose the namespace) the check is skipped gracefully.
• Static XBullAdapter.compareVersions(a, b) utility for semver-style comparison.

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How it was tested

• Existing tests updated: test/walletconnect.test.ts and test/paymentGraphChecker.test.ts
• New test files: test/freighterAdapter.test.ts, test/xbullAdapter.test.ts
• All acceptance criteria covered by dedicated test cases.
• 158 tests pass across 6 test files (vitest run).

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closes #771
closes #772
closes #773
closes #774

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