fix(core): quality audit — fix docs, validation, tests, and correctness

[AbdelStark]

• Fix README: correct license (MIT not Apache 2.0), fix broken quickstart
  (config.init → config.init_model), add architecture section, key concepts,
  status declaration, and development commands
• Fix lib.rs doc example to compile and run as a doctest
• Fix visualize_weights example: use op's trained RmsNorm instead of
  creating a fresh one (produced incorrect weight visualization)
• Add AttnResConfig::validate() with comprehensive checks (d_model > 0,
  num_layers even, d_model divisible by num_heads, dropout in range, etc.)
• Call validate() in init_model() to catch bad configs at initialization
• Add AttnResConfig::num_transformer_layers() helper
• Improve expect() messages in model forward pass
• Add 9 new tests: two-phase/standard equivalence, Full AttnRes boundary,
  RMSNorm 3D/4D consistency, config validation (good/bad), and more
• Update CLAUDE.md status from "pre-implementation" to "alpha"
• Add .env patterns to .gitignore

Total: 57 tests passing, clippy clean, fmt clean.

https://claude.ai/code/session_01NUqHj8Pz1bYNEtzyX3x3kW