add Config.BinaryPath, PATH auto-detect, and Variant() (#97, #98, #99)

[neverDefined]

Summary

• Config.BinaryPath opt-in override: absolute / relative / bare-name (resolved via PATH).
• PATH auto-detect when unset: bitcoind-inquisition first, falls back to bitcoind.
• Variant() getter parses getnetworkinfo.subversion once and caches under a mutex.
• Embedded bitcoind_manager.sh honors BITCOIND_BIN / BITCOIN_CLI_BIN env vars; defaults to literal names so direct invocation still works.

This is the foundation PR for Phase 6 — subsequent PRs add the deployment registry (PR2), worked example + README (PR3), and the time API (PR4).

Closes #97, #98, #99.

Test plan

• make ai-check green (fmt + vet + lint + test-race + vuln).
• New tests pass:
  • Test_Config_BinaryPath_Resolved — explicit BinaryPath = $(which bitcoind) starts/stops cleanly.
  • Test_Config_BinaryPath_Invalid — nonexistent path fails fast at New() with the path in the error.
  • TestRPC_Variant_Returns — running against Core resolves to VariantCore (not Unknown).
  • TestRPC_Variant_Cached — second call returns the same value after the node is stopped (proves caching).
  • TestRPC_Variant_PreStart — pre-Start() call returns errNotConnected.
  • TestRPC_Variant_StringRoundTrip — enum strings stable.
  • Test_Config updated to round-trip BinaryPath through Config().
• Reviewer-only: confirm against an Inquisition build that Variant() returns VariantInquisition (deferred to [Phase 6.E.4] TestVariantDetection smoke test #109 in PR5; out of scope here).

Notes

• Additive only — no breaking signatures, per CLAUDE.md invariant Config #5.
• Variant caching uses sync.Mutex + flag rather than sync.Once to avoid permanently caching a transient RPC error on the first call.
• bitcoin-cli is resolved as a sibling of bitcoind first, then via PATH — so Inquisition installs that ship paired binaries in their own directory work without further configuration.

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