Cleanup: drop dead engine flags from explorer

The ring-buffer engine has no stem-cache / sentinel / seam-log concept, so the ?debugSentinel, ?stems, ?stemLookahead, and ?seamLog flags (and the opts they fed exploreLive) were dead no-ops.

• Final verification of the whole rebuild: segment-parity byte-equal, engine.test ALL PASS, genre matrix diagonal-dominant 92/92, validate-genres --quick PASS, clicktest mode 1 clicks=0 (mode 3 clicks=0

▶ Play it: Rail Replacement Rapture — a bed-heavy genre where the found-sound layer carries the room https://stellate.app/?genre=transitwave&seed=4381&path=749.12536,768.12569,730.12569&xdur=13.122

b06b55f https://github.com/aboard-io/stellate/commit/b06b55fdd183788ff6dd8036a5f719484fd47c75