[Gear] Zatha'tek: model Necrotic Tear as a per-target debuff

[NN0323]

Summary

Fixes the one substantive issue found in code review of zathatek-breath-of-corruption: Necrotic Tear (spell 1305391) was modeled as a single player-wide buff instead of a per-target debuff.

In-game, Necrotic Tear is a stacking debuff applied to each target that persists until that target dies, granting +5% Breath of Corruption damage per stack (max 10). The implementation used make_buff(e.player, ...) — one shared stack counter — and amplified damage off that shared count regardless of which target was hit.

Why it mattered

Single-target results were correct, so this was silent in single-target (Patchwerk) smoke tests. But in multi-target / cleave / target-swap sims the shared player buff:

• let stacks built up on one target inflate Breath of Corruption damage dealt to a different target (composite_da_multiplier read the global stack count, not s->target's), and
• never reset when a target died (infinite-duration player buff), so stacks only ever ramped up — an overestimate in add-wave / DungeonSlice styles.

The fix

Convert Necrotic Tear to the per-target debuff infrastructure already used elsewhere in unique_gear_midnight.cpp (void_execution_mandate, torments_duality, potion_of_zealotry):

• ctor: target_debuff = e.player->find_spell( 1305391 );
• composite_da_multiplier: if ( auto debuff = find_debuff( s->target ) ) m *= 1.0 + debuff->check() * stack_mod;
• impact: get_debuff( s->target )->trigger();

get_debuff(t) lazily creates the debuff per target via make_buff(actor_pair_t(t, player), …, target_debuff), so max_stack (10) and the infinite duration are still inherited from the spell data and per-target stacks now reset on target death for free. Deliberately does not copy potion_of_zealotry's target-swap expiry logic — Necrotic Tear persists per target until death.

Also: const-qualify stack_mod, drop the now-redundant stack guard (the lazy find_debuff null check is the necessary one), and clarify the spell-id header comment (the +5%/stack value lives on the driver's effectN(2), not the tear spell).

Behavior impact

• Single-target: identical output — same spell data, same stacks/duration, same effectN(1) damage and effectN(2) +5%/stack.
• Multi-target / target-swap: corrected — stacks are now per-target and reset on death.

Verification

• Reviewed by three independent code-review agents; all converged on this as the single must-fix issue, with the rest of the implementation (RPPM 2.5 + haste scaling, damage/role_mult sourcing, max-stack/duration, constructor arg order) verified correct against the PTR DBC.
• The fix was adversarially re-verified against the engine API (action_t::target_debuff / get_debuff / find_debuff in action.hpp/action.cpp) and the neighboring precedents: API type-checks, single-target equivalence holds, impact ordering preserved, no dangling necrotic_tear references.
• Not locally compiled — no C++ toolchain was available on PATH in this environment (VS solution only). Change is static-analysis verified; a local/CI build is recommended before merge. Note the project builds with -Wall -Wextra -Wpedantic but not -Werror.

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