feat: add read-only RustChain event relay

[Scottcjn]

Summary

• Poll canonical GET /health, /epoch, and a bounded /api/miners page into deterministic state-change events.
• Add generation-qualified, cursor-resumable bounded MCP batches and long polling with explicit restart/reset and history-expiry semantics.
• Add a loopback-first SSE relay with bearer-protected remote opt-in, pre-thread connection limits, bounded memory/responses, backoff, and observable shutdown failures.
• Normalize volatile health counters and preserve node-provided miner totals without claiming a page length is global.
• Move balance reads to /wallet/balance while preserving canonical amount_rtc plus legacy balance, balance_rtc, and wallet_id aliases.
• Document honestly that MCP results are progressive bounded calls, not native partial-result streaming.
• Make core CI failures blocking while excluding only declared optional/legacy cases.

Closes #231.

Verification

• Focused relay, wallet, BoTTube, ecosystem, and signed-transfer suite: 116 passed
• Exact CI selection: 172 passed, 2 explicitly deselected
• ruff check and compileall passed
• Wheel build passed: rustchain_mcp-0.4.0-py3-none-any.whl
• git diff --check and commit whitespace checks passed
• Tests use fake nodes and loopback only; no live credentials, writes, or deployment