Layer 1.5: architecture-general neural existence proof

Re-scopes Layer 2 into a cheaper, architecture-general neural collapse proof
before the LLM rung. Realises the same Wright–Fisher abstractions in real trained
generative models on a fully-synthetic sandbox with an exact oracle, reusing
knowledge.metrics/truth/seeding and the output contract so neural curves overlay
the Layer-1 analytic curves.

  - src/neural/: synthetic token-grammar sandbox (lossless identity + stochastic
    style), ExactOracle, HistogramModel bridge, generation loop, experiment runner
  - HARD GATE passed: histogram lineage reproduces Layer 1 exactly (neutral decay,
    exact H_eq, tracks run_lineage) — tests/test_neural_validation.py
  - torch models: autoregressive RNN + MLP (VAE implemented, not yet fidelity-
    passing); determinism seeding derived from the SeedSequence stream
  - N0 bridge (neural g*=0.047 ≈ Layer-1 0.048), N1 collapse-in-weights, N2 phase
    boundary, N5 architecture-generality (collapse + grounding-rescue in histogram
    + RNN + MLP). Manifests/configs committed; parquet gitignored, hashes tracked
  - additive backward-compatible save_artifacts extension; Makefile neural targets

Finding: neural smoothing partially resists H-collapse, so forward-KL and tail
survival are the sharp neural collapse metrics (H is smooth, per Layer 1).

92 tests green. Remaining (tasks/todo.md): N4 merge, N2 refine, N3/N6, VAE
fidelity, MNIST tier, figures. LLM/LoRA rung and C3 deferred.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

results/N2/manifest.json

@@ -0,0 +1,17 @@
+{
+  "experiment": "N2_grounding_phase_boundary_neural",
+  "master_seed": 20260704,
+  "git_commit": null,
+  "python": "3.14.5",
+  "libraries": {
+    "numpy": "2.5.0",
+    "scipy": "1.18.0",
+    "pandas": "3.0.3",
+    "pyarrow": "24.0.0",
+    "torch": "2.12.1"
+  },
+  "rows": 1085,
+  "results_sha256": "93590677221955189814ccf044b9dc8aee36b026e350c9dea7e8d23b137e514b",
+  "layer": "1.5",
+  "model_kind": "rnn"
+}

results/N2/resolved_config.yaml

@@ -0,0 +1,305 @@
+experiment: N2_grounding_phase_boundary_neural
+seed: 20260704
+n_replicates: 5
+source_config:
+  experiment: N2_grounding_phase_boundary_neural
+  kind: gen_lineage
+  seed: 20260704
+  n_replicates: 5
+  generations: 30
+  synthetic:
+    K: 256
+    R: 1
+    tail: zipf
+    zipf_s: 1.3
+    tail_frac: 0.5
+    tail_threshold: 0.001
+    init: truth
+    style_len: 3
+    style_vocab: 5
+    id_base: 2
+  model:
+    kind: rnn
+    hidden: 128
+    embed: 24
+    epochs: 25
+    lr: 0.002
+    batch_size: 256
+    n_eval: 12000
+  dynamics:
+    n: 200
+    grounding:
+      m: 0
+      policy: proportional
+    remint:
+      enabled: false
+      period: null
+      H_gate: null
+  metrics:
+    kl_floor: 1.0e-09
+    support_eps: 1.0e-09
+  sweep:
+  - param: g
+    values:
+    - 0.0
+    - 0.005
+    - 0.01
+    - 0.02
+    - 0.05
+    - 0.1
+    - 0.2
+  output:
+    dir: results/N2
+grid:
+- label:
+    g: 0.0
+    m: 0
+  neural_cfg:
+    synthetic:
+      K: 256
+      R: 1
+      tail: zipf
+      zipf_s: 1.3
+      tail_frac: 0.5
+      tail_threshold: 0.001
+      init: truth
+      style_len: 3
+      style_vocab: 5
+      id_base: 2
+    model:
+      kind: rnn
+      hidden: 128
+      embed: 24
+      epochs: 25
+      lr: 0.002
+      batch_size: 256
+      n_eval: 12000
+    dynamics:
+      n: 200
+      grounding:
+        m: 0
+        policy: proportional
+      remint:
+        enabled: false
+        period: null
+        H_gate: null
+    generations: 30
+    metrics:
+      kl_floor: 1.0e-09
+      support_eps: 1.0e-09
+- label:
+    g: 0.005
+    m: 1
+  neural_cfg:
+    synthetic:
+      K: 256
+      R: 1
+      tail: zipf
+      zipf_s: 1.3
+      tail_frac: 0.5
+      tail_threshold: 0.001
+      init: truth
+      style_len: 3
+      style_vocab: 5
+      id_base: 2
+    model:
+      kind: rnn
+      hidden: 128
+      embed: 24
+      epochs: 25
+      lr: 0.002
+      batch_size: 256
+      n_eval: 12000
+    dynamics:
+      n: 200
+      grounding:
+        m: 1
+        policy: proportional
+      remint:
+        enabled: false
+        period: null
+        H_gate: null
+    generations: 30
+    metrics:
+      kl_floor: 1.0e-09
+      support_eps: 1.0e-09
+- label:
+    g: 0.01
+    m: 2
+  neural_cfg:
+    synthetic:
+      K: 256
+      R: 1
+      tail: zipf
+      zipf_s: 1.3
+      tail_frac: 0.5
+      tail_threshold: 0.001
+      init: truth
+      style_len: 3
+      style_vocab: 5
+      id_base: 2
+    model:
+      kind: rnn
+      hidden: 128
+      embed: 24
+      epochs: 25
+      lr: 0.002
+      batch_size: 256
+      n_eval: 12000
+    dynamics:
+      n: 200
+      grounding:
+        m: 2
+        policy: proportional
+      remint:
+        enabled: false
+        period: null
+        H_gate: null
+    generations: 30
+    metrics:
+      kl_floor: 1.0e-09
+      support_eps: 1.0e-09
+- label:
+    g: 0.02
+    m: 4
+  neural_cfg:
+    synthetic:
+      K: 256
+      R: 1
+      tail: zipf
+      zipf_s: 1.3
+      tail_frac: 0.5
+      tail_threshold: 0.001
+      init: truth
+      style_len: 3
+      style_vocab: 5
+      id_base: 2
+    model:
+      kind: rnn
+      hidden: 128
+      embed: 24
+      epochs: 25
+      lr: 0.002
+      batch_size: 256
+      n_eval: 12000
+    dynamics:
+      n: 200
+      grounding:
+        m: 4
+        policy: proportional
+      remint:
+        enabled: false
+        period: null
+        H_gate: null
+    generations: 30
+    metrics:
+      kl_floor: 1.0e-09
+      support_eps: 1.0e-09
+- label:
+    g: 0.05
+    m: 11
+  neural_cfg:
+    synthetic:
+      K: 256
+      R: 1
+      tail: zipf
+      zipf_s: 1.3
+      tail_frac: 0.5
+      tail_threshold: 0.001
+      init: truth
+      style_len: 3
+      style_vocab: 5
+      id_base: 2
+    model:
+      kind: rnn
+      hidden: 128
+      embed: 24
+      epochs: 25
+      lr: 0.002
+      batch_size: 256
+      n_eval: 12000
+    dynamics:
+      n: 200
+      grounding:
+        m: 11
+        policy: proportional
+      remint:
+        enabled: false
+        period: null
+        H_gate: null
+    generations: 30
+    metrics:
+      kl_floor: 1.0e-09
+      support_eps: 1.0e-09
+- label:
+    g: 0.1
+    m: 22
+  neural_cfg:
+    synthetic:
+      K: 256
+      R: 1
+      tail: zipf
+      zipf_s: 1.3
+      tail_frac: 0.5
+      tail_threshold: 0.001
+      init: truth
+      style_len: 3
+      style_vocab: 5
+      id_base: 2
+    model:
+      kind: rnn
+      hidden: 128
+      embed: 24
+      epochs: 25
+      lr: 0.002
+      batch_size: 256
+      n_eval: 12000
+    dynamics:
+      n: 200
+      grounding:
+        m: 22
+        policy: proportional
+      remint:
+        enabled: false
+        period: null
+        H_gate: null
+    generations: 30
+    metrics:
+      kl_floor: 1.0e-09
+      support_eps: 1.0e-09
+- label:
+    g: 0.2
+    m: 50
+  neural_cfg:
+    synthetic:
+      K: 256
+      R: 1
+      tail: zipf
+      zipf_s: 1.3
+      tail_frac: 0.5
+      tail_threshold: 0.001
+      init: truth
+      style_len: 3
+      style_vocab: 5
+      id_base: 2
+    model:
+      kind: rnn
+      hidden: 128
+      embed: 24
+      epochs: 25
+      lr: 0.002
+      batch_size: 256
+      n_eval: 12000
+    dynamics:
+      n: 200
+      grounding:
+        m: 50
+        policy: proportional
+      remint:
+        enabled: false
+        period: null
+        H_gate: null
+    generations: 30
+    metrics:
+      kl_floor: 1.0e-09
+      support_eps: 1.0e-09