docs: Add extraction plan for ESP32C3 TX Backpack standalone project

[snokvist]

Summary

This PR adds a comprehensive extraction plan document that outlines the strategy for converting the ESP32C3_TX_Backpack_via_USB environment from the upstream ELRS Backpack repository into a standalone PlatformIO project.

Key Changes

• New document: docs/extraction_plan.md (289 lines)
  • Complete inventory of source files, libraries, and dependencies actually used by the USB TX backpack environment
  • Detailed categorization of upstream-shared code vs. Waybeam-only additions
  • Identification of dead code (AAT/HeadTracker/Fusion modules) that can be safely excluded
  • Two viable extraction approaches (copy-and-trim vs. greenfield)
  • Phased implementation plan with 5 phases, each producing a buildable firmware
  • Backporting strategy for future upstream changes
  • Open questions and effort estimates

Notable Details

The document provides:

• Inventory breakdown: ~3,900 LOC across source, libraries, headers, and build helpers
• Waybeam-specific modifications: Identified in 6 files (wired CRSF, OLED dashboard, passthrough mode, MSP opcodes, button gestures, config persistence)
• Build pipeline: Documents the Python helper scripts and HTML assets required for WiFi OTA functionality
• Backporting guidance: Clear touch-points for integrating future upstream TX-backpack fixes while preserving Waybeam customizations
• Risk mitigation: Recommends Phase 0-2 (fork + strip + collapse) as a low-risk initial step with hardware verification before proceeding to cleanup phases

This document serves as a reference for executing the extraction and maintaining the standalone project long-term.

https://claude.ai/code/session_01LWjgKdbNQcgMDmynteDXBj

[snokvist]

Plan executed — see snokvist/esp32-supermini-projects#24 for the standalone project. Extracted from this repo at base commit 4b2fd92. Verified pio run produces firmware.bin within 80 bytes of the ESP32C3_TX_Backpack_via_USB baseline.

Pushed a follow-up commit to this PR adding a Status banner to the plan doc so it's clear it's historical record.

Phases 4-5 (further flag cleanup, release tag, deprecating the USB env in this repo) deferred to a follow-up — open a new PR when ready.