Add pseudo-hit support for ailment sources

[turlockmike]

Two PRs #2320 into reviewable pieces, as requested in review. the generic fake-hit mechanic, ahead of the Infernal Legion skill itself (follow-up PR).

Description of the problem being solved:

Some skills and items inflict an ailment "as though dealing" damage without actually hitting: the Infernal Legion supports and Catha's Brilliance (minions Ignite for % of their max life), Stone Greaves and Beira's Anguish (Ignited Ground igniting as though dealing % of max life), Lycosidae (Blocking Poisons as though dealing 100 Base Chaos Damage). Treating these as real hits — the approach in #2320 — inflates hit DPS and Full DPS, and lets on-hit ailment sources (e.g. a Poison support) scale off damage that is never actually dealt.

This adds a generic skillData.pseudoHitAilment flag for such sources. The notional damage runs the full hit pipeline (increases, conversion, gain-as-extra, crit) and feeds the one ailment the skill represents, but the hit outputs are zeroed afterwards so it contributes nothing to hit DPS or Full DPS. The gate lives in canDoAilment, which both calcAverageUnmitigatedSourceDamage and calcMinMaxUnmitigatedAilmentSourceDamage route through, so it behaves the same for every ailment and no on-hit ailment can ride the notional damage. (calcMinMaxPoiseSourceDamage shares the same gate, so a fake hit correctly contributes no stun/poise buildup either.)

The existing Ignite<Type>HitDamage OVERRIDE (used by Burning Runes) covers the other semantic — a final flat value that bypasses the pipeline and never crits — and stays untouched; a skill picks whichever matches its in-game behaviour.

No in-tree skill sets the flag yet; the Infernal Legion follow-up PR is the first user.

Steps taken to verify a working solution:

• Added three tests to spec/System/TestAilments_spec.lua: a pseudo-hit source deals no hit damage but keeps its full ailment; a 100% poison-on-hit source produces no poison off the notional damage; ailment magnitude still scales the pseudo-hit ailment.
• Verified headless that a real-hit and a pseudo-hit Fireball produce identical IgniteDPS while the pseudo-hit's AverageDamage/AverageHit/TotalDPS are 0.
• Regenerated ModCache — no diff (no parser changes in this PR).

Link to a build that showcases this PR:

N/A — no in-tree skill sets the flag yet; the follow-up Infernal Legion PR showcases it.

Before screenshot:

N/A — no behaviour change until a skill sets the flag.

After screenshot:

N/A