build-fbs uses system flatc and doesn't detect orphaned generated files

[oberstet]

Bug Description

The build-fbs recipe and the "Verify regenerated files match committed files" workflow step have two issues that together allow orphaned/leftover generated files to accumulate undetected:

1. Justfile bug: build-fbs uses system flatc instead of the bundled flatc from vendored flatbuffers
2. Workflow bug: The verification step doesn't clean before regenerating, so orphaned files survive and checksums match

Current Behavior

Problem 1: Wrong flatc binary

In justfile, build-fbs uses:

FLATC="flatc"  # System flatc - version may differ!                                                                          

But autobahn-python bundles (currently) flatc v25.9.23 in the vendored flatbuffers package. After install-tools runs, the bundled flatc is available at ${VENV_PATH}/bin/flatc.

Problem 2: Orphaned files not detected

The workflow does:

1. Compute checksums of committed files (includes orphaned files)
2. Run just build-fbs (only adds/overwrites, never deletes)
3. Compare checksums (orphaned files are in BOTH, so they match!)

This allows files from older schema versions to persist indefinitely.

Evidence

Currently, there are 2 orphaned files in src/autobahn/wamp/gen/wamp/proto/:

Orphaned File	Current Replacement	Issue
Kdf.py	KDF.py	Old flatc naming + different enum values
ChannelBinding.py	TLSChannelBinding.py	Enum renamed in schema

These were discovered by manually running rm -rf src/autobahn/wamp/gen && just build-fbs && git diff --stat:

src/autobahn/wamp/gen/wamp/proto/ChannelBinding.py | 8 --------                                                              
src/autobahn/wamp/gen/wamp/proto/Kdf.py            | 9 ---------                                                             
2 files changed, 17 deletions(-)                                                                                             

Root Cause

When the FlatBuffers schema evolves (enum renamed, type removed, etc.), flatc no longer generates the old files. But since build-fbs doesn't clean first by itself, the old files remain. The checksum verification passes because it compares "before regeneration" vs "after regeneration" - both contain the orphans.

Proposed Fix

1. Fix justfile build-fbs to use bundled flatc:

# Before                                                                                                                     
FLATC="flatc"                                                                                                                
                                                                                                                            
# After                                                                                                                      
FLATC="${VENV_PATH}/bin/flatc"                                                                                               

2. Fix workflow to clean before regenerating:

# Before                                                                                                                     
- name: Build FlatBuffers binary schema & Python wrappers                                                                    
run: just build-fbs                                                                                                        
                                                                                                                            
# After                                                                                                                      
- name: Build FlatBuffers binary schema & Python wrappers                                                                    
run: |                                                                                                                     
    just clean-fbs                                                                                                           
    just build-fbs                                                                                                           

Expected Outcome

After implementing these fixes:

1. The workflow will fail on the next run because clean-fbs removes orphaned files, causing checksum mismatch
2. This failure (!) proves the detection mechanism works
3. A follow-up commit with just clean-fbs && just build-fbs will fix the orphaned files
4. Future schema changes that remove types will be detected immediately

Todo

• Update build-fbs in justfile to use ${VENV_PATH}/bin/flatc
• Update workflow step to call just clean-fbs before just build-fbs
• (After CI fails) Run just clean-fbs && just build-fbs and then Git commit regenerated files

Checklist

• I have searched existing issues to avoid duplicates
• I have provided a minimal reproducible example
• I have included version information
• I have included error messages/logs