feat: Flash Courier — Splicer Terminal, Data Shards & Run Briefing engine

[Copilot]

Adds the Flash Courier mechanic: a narrative-driven "Run Briefing" layer that sits above the existing Card Forge and Missions systems. Players slot three Data Shards into a Splicer Terminal, trigger a compile animation, then receive a branching prose briefing driven by their Cover Identity, District, and shard selection — no action mini-game, no server round-trip.

Data layer (src/lib/flashCourier.ts)

• DataShard, NavDeckSlots, CompiledNavDeck interfaces
• 18 static shards across three kinds: Vector (approach), Ghost (camouflage), Payload (wild card modifier)

Narrative Router (src/lib/runBriefing.ts)

• resolveRunBriefing(deck: CompiledNavDeck): StoryNode — maps the 5 variables to district-specific prose + tag-driven inserts + archetype coda + 2–3 branching choices with stealthDelta / speedDelta / rewardDelta / riskDelta hints
• Uses canonical District type; falls back gracefully for future districts

Splicer Terminal (src/pages/flashCourier/SplicerTerminal.tsx)

• Drag-and-drop slot zones per kind; click-to-slot as keyboard/touch fallback
• useCompileAnimation hook: 4.2 s pseudo-terminal sequence (scrolling ICE-cracking log, progress bar, blinking cursor); timers and rAF cancelled on unmount via cleanup ref

Flash Courier page (src/pages/FlashCourier.tsx)

• Identity + District pickers sourced from player's existing card collection
• RunBriefingPanel renders prose + choice buttons with stat modifier chips; reset flow to re-compile

Wiring

• FLASH_COURIER feature flag added (default true)
• /flash-courier route registered behind ProtectedRoute
• ~400 lines of CSS appended to index.css using existing theme variables; prefers-reduced-motion overrides included