docs: update ember-client readme, add crates index, regenerate protos

[kacy]

summary

documentation catch-up for the gRPC API gap (pr #306) and client growth:

• ember-client readme — fully rewritten to cover all 65+ commands across every group (strings, keys, lists, hashes, sets, sorted sets, server, slowlog, pub/sub), public types (ScanPage, SlowlogEntry, Message, SimResult), the full pipeline method reference, and the vector feature section
• crates/README.md — new navigation index with a crate table (including the ember-core → emberkv-core package name note that trips people up), ascii dependency graph, cargo cheat-sheet, and makefile target reference including the proto-gen targets
• proto regeneration — Go and Python gRPC stubs updated from the latest proto definition; all 39 new rpcs from pr feat(grpc): close api gap — 39 new rpcs #306 are now reflected in the generated code
• ember-py Makefile fix — python → python3 for compatibility on systems where only python3 is in $PATH

what was tested

• make proto-go — generated clients/ember-go/proto/ember/v1/ember.pb.go and ember_grpc.pb.go cleanly
• make proto-py — generated clients/ember-py/ember/proto/ember/v1/ember_pb2.py and ember_pb2_grpc.py cleanly, import fix applied by sed
• cargo build -p ember-client passes (no code changes)
• verified all method names in the readme against the actual commands.rs, pipeline.rs, and vector.rs source