feat: add ship receipts app shell

[Pro777]

Summary

• add a dedicated SHIP.RECEIPTS StackChan app instead of wiring Ship Receipts semantics into core firmware
• introduce an app-local scene contract for card/avatar beats, metadata, sequencing state, and control commands
• add checked-in examples plus a tiny helper script for emitting supported scene/control payloads

Why this shape

Ship Receipts should live as an app on top of StackChan core.

Core stays responsible for:

• transport
• motion/audio/LED device capabilities
• diagnostics/logging
• generic renderer primitives

The app owns:

• scene semantics
• mode/content logic
• sequencing policy
• app-local commands
• Ship Receipts-specific orchestration

Included app-local capabilities

• scene fields: id, mode, title, line, speaker, presentation_type, visual_template, emotion, duration_ms, motion, led, play_notification, sticky
• sequence states:
  • DemoRotation
  • LiveOverride
  • LiveSticky
• control commands:
  • shipReceiptsResumeDemo
  • shipReceiptsShowStatus
  • shipReceiptsSetMode

Artifacts

• app shell and parser under firmware/main/apps/app_ship_receipts/
• example payloads under firmware/main/apps/app_ship_receipts/examples/
• helper script:
  • firmware/main/apps/app_ship_receipts/tools/emit_ship_receipts_scene.py

Notes

• this PR intentionally keeps Ship Receipts semantics app-local
• no firmware flash/live hardware verification was required yet for this architecture slice