Split model construction into measured stages

[7etsuo]

Summary

• split model validation and memory sizing into small checked stages
• build parameters through a named registry cursor without changing RNG or registry order
• measure and place activation and gradient arenas through explicit cursors, then reconcile every storage family at runtime
• strengthen Chapters 9 and 10, lab witnesses, boundary tests, and release-mode failure tests

Compatibility

One-step and 50-step checkpoints are byte-identical to merged commit a5b6979. Seeded sample output and diagnostics are also byte-identical. Exact commands and SHA-256 values are recorded in EVIDENCE.md.

Validation

• make OPENMP=0 check-all
• make OPENMP=1 check-parallel
• make check-sanitizers
• GCC and Clang focused model checks
• NDEBUG invalid-construction checks
• make check-book
• python3 scripts/check-graph.py
• sha256sum --check --strict --quiet EVIDENCE.sha256
• git diff --check
• independent code and documentation reviews

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