feat(game): skills.ts — XP, levels, power bands, titles (#45)

lib/game/skills.ts

@@ -0,0 +1,53 @@
+// Pure skill-progression module (spec §6): XP accrual, level→power, titles. No lib/db imports — operates on plain SkillRow data (Task 12's crew_skills table); the frontend imports it for the CREW-tab display.
+import type { SkillName } from './crew-costs'
+import { POWER_BANDS, XP_THRESHOLDS } from './crew-costs'
+export interface SkillRow {
+  saveId: string
+  crewId: string
+  skill: SkillName
+  level: number
+  xp: number
+}
+export type CrewIdentity = { name: string; role: string }
+// Highest level 0–5 with xp ≥ XP_THRESHOLDS[level−1]; thresholds [5,10,15,20,25], cap 5.
+export function levelFromXp(xp: number): number {
+  let level = 0
+  for (let l = 1; l <= XP_THRESHOLDS.length; l++) if (xp >= XP_THRESHOLDS[l - 1]) level = l
+  return level
+}
+// L0–2 → 1 · L3–4 → 2 · L5 → 3 (direct lookup into crew-costs POWER_BANDS).
+export function powerForLevel(level: number): number {
+  return POWER_BANDS[level]
+}
+// From the member's HIGHEST skill level: ≥5 → "MASTER <ROLE>", ≥3 → "SENIOR <ROLE>", below → null.
+export function titleFor(member: CrewIdentity, skillRows: SkillRow[]): string | null {
+  const highest = skillRows.reduce((max, r) => Math.max(max, r.level), 0)
+  if (highest >= 5) return `MASTER ${member.role}`
+  if (highest >= 3) return `SENIOR ${member.role}`
+  return null
+}
+// +1 XP to every listed member's row for `skill`; returns updated rows + any level-up events. An event
+// fires when the gained skill steps into a titled band (3 → SENIOR, 5 → MASTER), e.g. "BEN OKONKWO is now a SENIOR PILOT — NAVIGATION L3" (title from the member's HIGHEST skill).
+export function grantXp(
+  skillRows: SkillRow[],
+  crewIds: string[],
+  skill: SkillName,
+  members: Record<string, CrewIdentity>,
+): { rows: SkillRow[]; events: string[] } {
+  const rows = skillRows.map((r) =>
+    r.skill === skill && crewIds.includes(r.crewId)
+      ? { ...r, xp: r.xp + 1, level: levelFromXp(r.xp + 1) }
+      : r,
+  )
+  const events: string[] = []
+  for (const crewId of crewIds) {
+    const member = members[crewId]
+    if (!member) continue
+    const newLevel = rows.find((r) => r.crewId === crewId && r.skill === skill)?.level ?? 0
+    if (newLevel === 3 || newLevel === 5) {
+      const title = titleFor(member, rows.filter((r) => r.crewId === crewId))
+      if (title) events.push(`${member.name} is now a ${title} — ${skill} L${newLevel}`)
+    }
+  }
+  return { rows, events }
+}