Engine: add flat (no-dice) heal formula support; retire Wholeness of Body's thin ACTION_EFFECT_FN workaround (#1501)

[Sandersland]

Follow-up from #1501 (2014 Way of the Open Hand), merged to staging 2026-08-04.

Why

2014 Wholeness of Body heals a flat amount — 3 × monk level, no dice. The effect-resolution engine has no "flat, no-dice" heal formula, so #1501 worked around it with a thin, fallow-ignore-suppressed DERIVED_ACTIONS key + ACTION_EFFECT_FN.wholenessOfBodyAction entry (a near-duplicate of the 2024 sibling) plus a frontend healRoll reading the monk class entry's own level.

Two symptoms:

• The heal renders as a 0d1 + 18 roll label (correct total for a L6 monk, but non-idiomatic on-screen — flagged in 2014 Monk (3/7): Way of the Open Hand — Wholeness of Body, Tranquility, 2014 Quivering Palm #1733 review as a low-severity cosmetic).
• A new ACTION_EFFECT_FN entry was added where the engine-first roadmap (Epic: Declarative subclass engine + homebrew (roadmap) #416) wants pure seed data — justified only by this gap.

Scope

• Add a flat/no-dice heal formula to the effect engine (a RollSpec-shaped { flat: N } or { count: 0 }-clean-label path — settle the shape), so a fixed heal amount resolves and renders without a 0d1 artifact.
• Retire wholenessOfBodyAction's workaround ACTION_EFFECT_FN + its fallow-ignore suppression once the engine supports the flat form; move Wholeness of Body onto pure seed data.
• Fix the roll-label rendering so a flat heal shows just the number (e.g. "18"), not "0d1 + 18" — guard formatRollSpec for count=0 or use the new flat shape.

Relevant code

backend/src/lib/classes/actions.ts (ACTION_EFFECT_FN.wholenessOfBodyAction, DERIVED_ACTIONS), the frontend healRoll/formatRollSpec in turnOptions.ts, the effect-resolution engine (RollSpec).

Dependencies

Part of the declarative subclass/effect engine roadmap #416. Not a 2014-gate (#1372) blocker — the workaround is correct, just not clean.