feat(example): generic test-matrix commands (sp_set/sp_setpc/sp_setent + sp_help)

Replace the one-command-per-feature demos with generic commands that exercise
every encoder type (int/uint/float/bool/vec/string/bytes) across every mode
(uniform / per-client / per-entity) against any field, no recompile. Add sp_help
listing the grid + examples; keep sp_fakehp/sp_fakename presets + the read-only
serializer probe. README example section rewritten to match.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>