[Phase 6.E.4] TestVariantDetection smoke test

[neverDefined]

Why

Smoke test that Variant() (#A3) actually returns the correct enum against whatever bitcoind happens to be on PATH. Makes regressions in subversion parsing fail loudly.

Proposed change

In examples_inquisition_test.go (or a new file): TestVariantDetection:

1. Start a node.
2. Call Variant(); assert it's VariantCore or VariantInquisition (not VariantUnknown).
3. Log the value with t.Logf so CI output shows which binary the suite ran against.

Port 19852.

Acceptance criteria

• Passes on Core (returns VariantCore).
• Passes on Inquisition (returns VariantInquisition).
• VariantUnknown is treated as a test failure.

Notes

Depends on #A3. Closes Phase 6.1 alongside #E3 (PR5).

[neverDefined]

Closed via #116.