[Vengeance] Per-fragment activation timing for Fracture

[taherbert]

Fragment activation timing calibrated against 5,200+ WCL events across 3 VDH dungeon runs. Fracture fragments spawn at the player, not the target, so they need their own timing instead of the shared Gaussian.

Changes

Vengeance default Gaussian

gauss<760, 120>() → gauss<830, 80>(). WCL shows ~830-864ms for damage-tick sources. Tighter cluster than previously modeled.

Fracture per-fragment timing

Adds optional activation_delay to spawn_soul_fragment() / soul_fragment_t that bypasses the shared Gaussian in get_travel_time(). Default sentinel (timespan_t::min()) preserves existing behavior for all other call sites.

Fragment spawning moves from fracture_damage_t::impact() to fracture_t::impact().

Aldrachi Tactics fragment delay

AT arrives at ~940ms (no Meta) / ~1065ms (Meta). Added Vengeance-specific spawn delay (110ms / 235ms) before the default Gaussian. Havoc/Devourer unchanged.

Timing summary

Fragment	WCL observed	Old model	New model
Fracture MH	641ms	0 + gauss(760) = 760ms	gauss<641, 10>
Fracture OH	933ms	150ms + gauss(760) = 910ms	OH delay + gauss<783, 20>
Fracture Meta +1	~1000ms	500ms + gauss(760) = 1260ms	170ms spawn + gauss<830, 80>
AT (no Meta)	~940ms	0 + gauss(760) = 760ms	110ms spawn + gauss<830, 80>
AT (Meta)	~1065ms	0 + gauss(760) = 760ms	235ms spawn + gauss<830, 80>

Other sources (unchanged, validated against WCL)

These all fit the shared Gaussian. No per-source overrides needed.

Source	WCL observed (ms)	Notes
Fallout	858	Initial burst only, all frags arrive on same tick
Sigil of Spite	830	After activation delay (1s Quickened / 2s base)
Soul Carver	862	First frag at server tick; 3 "immediate" arrive ~150ms apart
Fel Devastation	864 (tick 1), 1707 (tick 2)	Meteoric Rise only (n=31, one player). Mean 2.48 frags/cast. tick 3 likely clipped by 2s channel ending.