fuzzing plus staticcheck on CI

[m0wfo]

Added 4 fuzz targets with seed corpora, plus a checkInvariants helper:

• FuzzGetReturnsBackendAndDeterministic Get returns a real backend, same key → same result
• FuzzAddRemoveRoundTrip remove + re-add a backend restores the exact mapping
• FuzzSetOrderIndependent same backends in any order produce identical tables
• FuzzAddKeepsInvariants Add leaves the lookup table valid (full, in-range, every backend present)

Also added staticcheck run during CI, and fixed 1 issue- bigM constant no longer used