test: commit fuzz corpus under testdata/fuzz/

[millerjp]

Summary

FuzzLoadStore and FuzzConcurrent have inline f.Add seeds but no testdata/fuzz/FuzzXxx/ directory. Interesting inputs found during fuzzing sessions are lost between contributors. A committed corpus lets CI replay those inputs deterministically.

Scope

• Mirror each f.Add entry as a file under testdata/fuzz/FuzzLoadStore/ and testdata/fuzz/FuzzConcurrent/.
• Document the procedure for adding new corpus entries (after a local fuzz session finds a crash, copy from $GOCACHE/fuzz/).

Acceptance criteria

1. Directories exist with at least the current f.Add entries as files.
2. CI test job runs fuzz targets in -run-only mode and the corpus files are exercised.

Source: go-quality agent.