feat(wallet): add SigmaJS prover

[TerraTectra]

Summary

• add a SigmaJS-backed ISigmaProver implementation to @fleet-sdk/wallet
• reduce and sign transactions protected by arbitrary ErgoTree scripts
• support Fleet and EIP-12 unsigned transaction objects, explicit token burning, network and base-cost options
• sign and verify Sigma proposition message proofs through SigmaJS
• add documentation, package dependency and release changeset

Validation

• pnpm test:unit — 67 files, 1,142 tests passed
• pnpm cov:check — 100% statements, functions and lines; new prover has 100% branch coverage
• pnpm test:lint — passed
• pnpm -r build — passed
• pnpm --filter @fleet-sdk/wallet build — passed without warnings

Closes #110

[changeset-bot]

🦋 Changeset detected

Latest commit: 4310ceb

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@fleet-sdk/wallet	Minor
@fleet-sdk/mock-chain	Minor

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[ArgosOff]

@arobsn @mhrn83 could you please review this submission for issue #110 and confirm the 50 ERG bounty status? The PR is currently mergeable. It adds the SigmaJS prover, transaction and message signing tests, documentation, and a changeset. I am available to address review feedback promptly here in writing.

[ArgosOff]

Local verification update: branch is 0 commits behind master and mergeable. Recursive workspace build passes, exit 0. Lint passes across 218 files and format/import checks pass across 220 files. New SigmaJS prover tests pass 10/10. Targeted V8 coverage for sigmaJsProver.ts is 100% statements, branches, functions, and lines. Full unit run reached 1141/1142; the only failure was the unchanged property test in proveDLogProtocol.spec.ts timing out at 5 seconds under full-suite load. Its immediate isolated rerun passed 8/8 in 3.82 seconds, including the timed-out test in 2.56 seconds. No file involved in that transient timeout is modified by this PR. Ready for review.