ltc: saturate emergency-decay target shift to prevent uint256 wrap - #561
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Step 3 of compute_share_target used a bare prev<<halvings on the 256-bit target. With a large prev and a multi-window share gap the shift overflows 256 bits and wraps to a tiny value (a HARDER target), accelerating the very death spiral the emergency decay exists to arrest. The old halvings<256 guard bounded shift width only, never magnitude. Add emergency_decay_shl() saturating helper (clamps to max_target on overflow) and a 5-case conformance KAT enforcing the easing-only invariant (emergency decay never decreases target).
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Replaces the bare
prev << halvingsin compute_share_target Step 3 with a saturatingemergency_decay_shl()helper that clamps to max_target on 256-bit overflow. A large prev across a multi-window share gap could previously wrap to a tiny (harder) target and accelerate a death spiral; this makes the emergency-decay path easing-only.Scope: src/impl/ltc/ only (single-coin, LTC lane). DOGE rides the same site; BTC carries its own copy of the helper separately in its lane (no cross-coin bundling).
Tests: new 5-case conformance KAT (easing-only invariant) green; full ltc share_test 11/11 green; clean rebuild.
Head: da75622. Held for integrator merge tap (consensus-bearing path).