feat(stats): make milestone computation pluggable via strategy

[Amund211]

Summary

Extracts the milestone-value selection in computeStatProgression into a pluggable MilestoneStrategy — (value, trendingUpward, stat) => number — while leaving the time-to-reach math (cubic solver for index, linear algebra for ratios, etc.) untouched.

The built-in nextNaturalMilestone reproduces today's behavior exactly: next prestige for stars, next whole number for fkdr/kdr, next round number scaled to order of magnitude for everything else. It's wired as an optional trailing param on computeStatProgression (defaulting to nextNaturalMilestone), so callers can swap in a different scheme without touching the solvers.

Contract: a strategy must return a value strictly greater than value when trending up, and strictly less when trending down — the solvers rely on this to keep daysUntilMilestone non-negative and correctly signed.

Why

We want to be able to choose different milestone schemes in different places — in particular snappier, finer-grained milestones, which we'll use in #256. This PR is just the pluggable seam (the default preserves current behavior); the alternate "snappier" strategy and where it gets selected land in #256.

Tests

Built red-green; two new test families in progression.unit.test.ts:

• nextNaturalMilestone unit tests — stars/quotient/round-number, up & down, power-of-ten boundaries, non-positive clamp, and a strict-monotonic contract check.
• Strategy injection tests on computeStatProgression — assert the injected strategy receives the expected (value, trendingUpward, stat) per stat type (including the dividend key finalKills in the infinite-ratio path), and that its returned milestone is respected for simple linear / quotient / index / stars cases (with daysUntilMilestone recomputed accordingly).

pnpm test:unit → 562 passing (34 new; the prior suite is unchanged and green, confirming the default preserves behavior). pnpm tsc and oxlint/oxfmt clean on the changed files.

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[Copilot]

Pull request overview

This PR makes milestone target selection in computeStatProgression pluggable via a MilestoneStrategy callback, while keeping the existing time-to-reach solvers unchanged. It introduces a default nextNaturalMilestone strategy intended to preserve existing milestone behavior and adds unit tests covering both the default strategy and strategy injection.

Changes:

• Added MilestoneStrategy and default nextNaturalMilestone, and threaded an optional strategy parameter through computeStatProgression (and internal quotient/index paths).
• Replaced hard-coded milestone selection logic for quotient, index, stars, and linear counters with calls to the strategy.
• Added new unit tests for nextNaturalMilestone and for strategy injection behavior in computeStatProgression.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/stats/progression.ts	Introduces MilestoneStrategy/nextNaturalMilestone and applies the strategy throughout milestone selection in progression computations.
src/stats/progression.unit.test.ts	Adds unit tests for the default milestone strategy and for injected-strategy behavior in computeStatProgression.

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