refactor(stats): centralise good/bad trend logic in format.ts

[Amund211]

What

Follows the pattern of #316 (which introduced src/stats/format.ts as the single home for stat-kind formatting knowledge) by moving the good/bad stat functionality into that same shared helper file, and consolidating the trend logic that was open-coded in two places on the session page.

Moved into src/stats/format.ts

• BAD_STATS + isBadStat(stat) — previously a module-level constant in src/routes/session/$uuid.tsx. This is stat semantics (which stats get error colours when rising), so it now sits next to STAT_FORMAT_KIND.
• getTrendDirection(from, to) — the three-way up/down/flat numeric comparison, which was open-coded as an identical if/else chain in both SessionStatCard and ProgressionValueAndMilestone.
• getTrendSentiment(stat, direction) — good/bad/neutral, folding in isBadStat. This is the (direction, isBad) → good/bad rule that was duplicated (as an inline (dir === "up") === badStat expression) at both call sites.

Call sites

SessionStatCard and ProgressionValueAndMilestone each did the same two steps — compare two numbers into a direction, then map (direction, isBad) to a MUI colour. Both now:

const direction = getTrendDirection(from, to);
const color = SENTIMENT_COLOR[getTrendSentiment(stat, direction)];

The MUI-specific sentiment → color mapping (good → success, bad → error, neutral → undefined) is a single module-level SENTIMENT_COLOR record in the route (kept out of format.ts so the shared file stays free of MUI). The change is behaviour-preserving — same colours, same icons.

Other consolidation candidates (left out, deliberately)

• isLinearStat / getRelatedStats — pure stat metadata, but each is used in only one file ($uuid.tsx), so moving them would be relocation without de-duplication. Happy to move them if you'd prefer all stat metadata to live in format.ts.
• ProgressionCaption's inline .toLocaleString(undefined, { min/maxFractionDigits: 2 }) — repeated ~20× for per-day rates. These are rate values, not stat values, so they don't map to a StatKey format kind; unifying them is a larger refactor and there's already a TODO on the quotient cases. Out of scope here.

Test plan

• pnpm tsc + pnpm tsc:node — clean
• pnpm lint:check (oxlint + oxfmt) — clean
• pnpm test:unit — 786 pass (incl. new src/stats/format.unit.test.ts covering isBadStat / getTrendDirection / getTrendSentiment)
• pnpm test (browser) — 987 pass

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[Copilot]

Pull request overview

Refactors session-page stat trend rendering by centralizing “bad stat” semantics and trend direction/sentiment logic into src/stats/format.ts, aligning with the earlier formatting consolidation in #316.

Changes:

• Added BAD_STATS, isBadStat, getTrendDirection, and getTrendSentiment to src/stats/format.ts.
• Updated session route components to use shared trend direction/sentiment helpers plus a local (MUI-only) SENTIMENT_COLOR mapping.
• Added unit tests covering bad-stat classification and trend direction/sentiment helpers.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/stats/format.ts	Centralizes bad-stat semantics and shared trend direction/sentiment helpers.
src/routes/session/$uuid.tsx	Replaces duplicated trend logic with shared helpers and a route-local sentiment→MUI-color map.
src/stats/format.unit.test.ts	Adds unit coverage for isBadStat, getTrendDirection, and getTrendSentiment.

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