Circular import: synaptic_plasticity_hebbian cannot be imported first

[cdeust]

Reproduction

$ python -c "import mcp_server.core.synaptic_plasticity_hebbian"
ImportError: cannot import name 'apply_hebbian_update' from partially
initialized module 'mcp_server.core.synaptic_plasticity_hebbian'
(most likely due to a circular import)

mcp_server/core/synaptic_plasticity_hebbian.py does
from mcp_server.core.synaptic_plasticity import _MAX_WEIGHT, _MIN_WEIGHT
while synaptic_plasticity.py imports apply_hebbian_update back from it
(# noqa: E402 site). Importing the parent module first resolves the cycle,
which is why the suite stays green — but any consumer (REPL, script, future
module) that imports the _hebbian module directly fails.

Surfaced during the #197 family-3 (PLR2004/E501) sweep, which touched the
file but deliberately did not restructure imports (behavior-preserving
mandate). Deferred per coding-standards §14.3: outside that change's blast
radius — no test, build, or lint step the sweep triggers fails on it.

Acceptance criteria

1. python -c "import mcp_server.core.synaptic_plasticity_hebbian" succeeds
   in a fresh interpreter (regression test asserting exactly that, run in a
   subprocess so no prior import masks the cycle).
2. The weight-bound constants live in exactly one module with no
   back-import cycle (e.g. hoisted to a leaf module both files import).
3. Existing suite passes unchanged.

[cdeust]

The #197 family-4 (PLC0415) sweep's per-module import verification confirms a second member of this cycle family: mcp_server.core.synaptic_plasticity_stochastic also cannot be imported first (ImportError: cannot import name 'apply_stochastic_hebbian_update' from partially initialized module, fresh interpreter, main 3a1ba1e, 2026-07-28) — it imports from synaptic_plasticity_hebbian, which trips the same synaptic_plasticity back-import documented here. Whichever restructuring closes this issue should add the stochastic module to criterion 1's subprocess regression test. The consolidation-package instance of the same defect class is tracked separately as #237.