Use TFUNIPAYLOAD01 build workflow and move matrix into fw/

[kaklik]

Motivation

• Keep the firmware CI identical to the working TFUNIPAYLOAD01 workflow to remove unexplained divergences in build_fw.yml.
• Load the build matrix from the fw/ tree so firmware targets live next to their projects and the workflow can read fw/build_matrix.yaml directly.
• Make project PlatformIO config canonical by pointing per-project platformio.ini entries at the shared TFUNIPAYLOAD01 configuration.

Description

• Replaced .github/workflows/build_fw.yml with the exact content provided (job renames, trigger filters, matrix loading and commit logic preserved) and added a load-matrix job that reads fw/build_matrix.yaml.
• Added fw/build_matrix.yaml (YAML list of AIRDOS03 targets) and removed the previous matrix file from .github/workflows/ by moving it under fw/.
• Created symlink-style platformio.ini entries for fw/AIRDOS03_MAVLink and fw/AIRDOS03_USTDFF that point to the canonical ../../hw/modules/TFUNIPAYLOAD01/fw/platformio.ini, moved .ino files into src/main.cpp, added #include <Arduino.h> where appropriate, and updated githash.h handling and output artifact paths in the workflow.
• Updated fw/README.md to document the PlatformIO project layout and local build/flash commands, and added a fw/boards symlink to the canonical boards folder.

Testing

• Ran git diff --check to ensure no whitespace or index errors and it passed.
• Parsed the workflow YAML with ruby -e 'require "yaml"; YAML.load_file(".github/workflows/build_fw.yml")' to validate syntax and it passed.
• Parsed fw/build_matrix.yaml with ruby -e 'require "yaml"; d=YAML.load_file("fw/build_matrix.yaml")' to confirm the matrix loads and it passed.
• Verified repository status with git status --short and committed the changes; the commit was created successfully and the PR was opened via the make_pr helper.

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Codex Task