Fixes #2389.

Description of the problem being solved:

Verglas exports its 1% Damage gained as extra Cold per 2,000 maximum Life of the destroyed Ice Crystal and its 6-second duration, but SupportVerglasPlayer had no stat mapping. As a result, linking or disabling Verglas produced identical per-skill and Full DPS values.

This adds:

• A local DamageGainAsCold modifier for skills actually supported by Verglas.
• A default-off configuration for destroying an Ice Crystal in the past 6 seconds.
• Automatic maximum-Life detection from enabled Ice Crystal skills in the build, using the highest calculated value.
• An optional Life override for a different crystal source or a specific lower-Life crystal.
• Full-DPS cache invalidation when the resolved cross-skill Ice Crystal Life changes.
• Display-only handling for the exported buff duration.

The modifier uses the existing Multiplier tag with div = 2000, which floors to complete 2,000-Life increments. It is intentionally not a GlobalEffect, since Verglas only applies to supported skills.

The automatic value is shown as the input placeholder. Leaving the field empty uses that value; entering a positive value takes explicit precedence. The checkbox remains default-off because having an Ice Crystal skill does not imply that the player destroyed a crystal within the past 6 seconds.

Steps taken to verify a working solution:

• Focused Verglas regression: 1/1 passed.
• Complete TestSkills_spec.lua: 61/61 passed with 0 failures and 0 errors.
• Complete TestFullDPSCache_spec.lua: 8/8 passed with 0 failures and 0 errors.
• Regression covers the disabled condition, automatic Life and placeholder, manual override precedence, 2,000 / 3,999 / 4,000 Life breakpoints, 271,990 Life producing 135%, linked-versus-unlinked skill scope, selecting a higher-Life Ice Crystal source, cross-skill Full-DPS cache invalidation, and Full DPS returning to baseline when the condition is disabled.
• Affected build on this PR alone: automatic Frost Wall Life 30,908 and 26,087,034.64 Full DPS. The separate Frost Wall quality mapping fix in Fix Gemling Frost Wall Ice Crystal life #2386 is required for Advanced Thaumaturgy's additional quality to affect that Life.
• Affected build with Fix Gemling Frost Wall Ice Crystal life #2386 and this PR combined: automatic Frost Wall Life 271,990.4 and 47,163,971.67 Full DPS, matching the prior 135% gained-as-Cold proxy.
• Lua syntax checks and git diff --check passed.

Link to a build that showcases this PR:

https://pobb.in/CI4hChFipObn

Before screenshot:

Verglas enabled or disabled: 23,452,423.18 Full DPS.

After screenshot:

With "Ice Crystal destroyed (past 6s)?" enabled and the Life override left empty, Verglas uses the highest calculated Life from enabled Ice Crystal skills. With #2386 also applied, this build resolves 271,990.4 Life and 47,163,971.67 Full DPS.