Internalize NeuromodNeuron as router-internal type (no neuromod dependency)

[rmems]

Context

synaptic-mesh contains src/neuromod.rs which defines a NeuromodNeuron type (NIF — Neuromodulatory Integrative Fixed-threshold) with gain control. It is used only by ChannelRouter in src/router.rs.

The top-level pub mod neuromod is misleading — it implies synaptic-mesh owns general neuron models, when its scope is topology / wiring / delay / CSR / router.

Decision

synaptic-mesh will NOT add neuromod as a dependency. The two crates are kept independent so each can evolve without coupling. NeuromodNeuron stays in synaptic-mesh but is repositioned as a router-internal integration primitive, not a general neuron model.

See LIM-48 for the full decision record.

Goal

Internalize NeuromodNeuron as a router-internal type and document the crate boundary:

1. Move NeuromodNeuron from src/neuromod.rs → into src/router.rs (or src/router/ submodule).
2. Remove pub mod neuromod and pub use neuromod::NeuromodNeuron from src/lib.rs.
3. If external consumers need the type, re-export from router (pub use router::NeuromodNeuron); otherwise pub(crate).
4. Document crate boundary in src/lib.rs module docs and README.md.

Boundary

Crate	Owns
neuromod	Canonical neuron models — LIF, Izhikevich, Hodgkin-Huxley, GIF, FitzHugh-Nagumo, Lapicque
synaptic-mesh	Topology, wiring, delay, CSR sparse maps, ChannelRouter, router-internal NIF primitive

Non-goals

• Do NOT add neuromod as a dependency to synaptic-mesh.
• Do not change ChannelRouter routing logic or SynapticMesh propagation semantics.
• Do not merge synaptic-mesh and neuromod.
• No changes to SAAQ math, heartbeat, plasticity, fatigue, or STDP.

Acceptance criteria

• src/neuromod.rs is removed; NeuromodNeuron lives inside the router module.
• pub mod neuromod is removed from src/lib.rs.
• synaptic-mesh does not depend on neuromod in Cargo.toml.
• Crate boundary documented in src/lib.rs module-level docs and README.md.
• cargo build and cargo test pass.
• No router or propagation behavior changes (pure structural move).

Related

• Linear: LIM-48
• Extract AHL router into an adapter or feature-gated module #4 (LIM-28) — resolved by PR feat(router): neuromodulatory adaptation — dopamine, cortisol, serotonin, fatigue, plasticity #8 (generalized AhlRouter → ChannelRouter + neuromodulatory adaptation)
• Fix failing layered_mesh_feed_forward test (blocks CI in #10) #9 — was a duplicate of this issue; repurposed to fix the failing layered_mesh_feed_forward test

Suggested branch

refactor/synaptic-mesh-neuromod-boundary

[rmems]

Decision update (2026-07-02)

Resolution path changed: synaptic-mesh will NOT add neuromod as a dependency.

Why

The two crates are intentionally kept independent so each can evolve without coupling. Adding neuromod as a dep would pull in rand, optional sentry, and the full neuron-model surface area that synaptic-mesh doesn't need — it only uses the small NIF (Neuromodulatory Integrative Fixed-threshold) primitive for ChannelRouter integration.

What changes instead

NeuromodNeuron stays in synaptic-mesh but is repositioned as a router-internal integration primitive:

• Moved from src/neuromod.rs → into src/router.rs (or src/router/ submodule)
• pub mod neuromod removed from src/lib.rs
• Crate boundary documented: neuromod owns canonical neuron models; synaptic-mesh owns topology/wiring/delay/CSR/router and its internal NIF primitive

Full decision record: LIM-48

Implementation plan

• PR 1 (chore: license + CI + quality): closes chore(license): Switch from GPL-3.0-or-later to dual MIT/Apache-2.0 #11, Add GitHub Actions CI workflow for code validation #10, Fix failing layered_mesh_feed_forward test (blocks CI in #10) #9 (repurposed)
• PR 2 (refactor: internalize NeuromodNeuron): closes this issue — structural move, no behavior change