feat(builder): composable scale node for per-type damage transforms

[yipe]

What & why

Adds a general, composable scaling primitive to the builder so a sub-roll can be halved / doubled / dropped and still combine with plain rolls into one payload — without the flat-config .plus() merge silently dropping it (the limitation HalfRollBuilder hits today).

This is the building block for per-damage-type enemy resistance / immunity / vulnerability in the DPR calculator (yipe/dpr): build one sub-roll per damage type, scale (or omit) each, and sumRolls them into a single hit/crit/save payload. Because scaling stays per-type, floor(½) is applied per type (never to the merged total), and it flows into both the PMF and the rendered expression.

API

• ScaleNode AST node: { type: 'scale', numerator, denominator, rounding, child }, wired into resolve() (PMF), findDie(), getASTSignature(), and the expression renderer.
• RollBuilder.scaleResult(numerator, denominator = 1, rounding = 'floor') → ScaleRollBuilder. Renders N * (expr), (expr) // D, or (expr) * N // D.
• sumRolls(parts) — additive composite whose PMF convolves the parts and whose expression joins them with +, preserving parts that carry hidden state (scaleResult / half).

Examples:

• resistance → d6.scaleResult(1, 2) → (1d6) // 2, floored PMF {0,1,1,2,2,3}
• vulnerability → d6.scaleResult(2) → 2 * (1d6), mean 7
• sumRolls([base, fire.scaleResult(1, 2)]) → 1d8 + 3 + (1d6) // 2

Testing

New scale.test.ts covers floor/round/ceil scaling, the ///* rendering, the mixed-type per-type floor, and a full attack carrying resistance through both hit and crit payloads with attribution intact. Full suite green (yarn build && yarn typecheck && yarn test, 1230 tests).

Notes

• Existing flat .plus() hot path and the half node are untouched.
• The final commit vendors the built dist/ on this branch so consumers can install it as a git dependency before 0.4 is published to npm; revert once @yipe/dice@0.4 is published.