TC: tonic AP peak voltage and AHP depth outside thalamic-relay literature

[JCorson]

Summary

The THALAMIC_RELAY (TC) preset's tonic AP shape diverges from values
reported by McCormick & Huguenard (1992). The intra-burst AP shape
during the LTS rebound burst is within range — only the tonic-mode
shape is affected. Tests in tests/integration/test_thalamic_relay.py
are marked xfail(strict=True) until the underlying biology is fixed.

Failing assertions

Stimulus: 1 µA/cm² × 200 ms (tonic mode, ~9 Hz repetitive firing).

Metric	Model	Literature	Test
Mean tonic peak voltage	~+49 mV	+10 to +40 mV	test_tc_ap_peak_voltage_in_tc_tonic_range
Mean tonic AHP depth	~−77 mV	−55 to −75 mV	test_tc_ap_ahp_depth_in_tc_tonic_range

Half-width (~0.56 ms), threshold (~−46 mV), and tonic firing rate
(~9 Hz) are all within range. Burst structure (3–7 spikes, 200–500 Hz)
is already pinned by test_thalamic_relay_step_release_produces_multi_spike_lts_burst
and unchanged.

Hypotheses

Same kinetic pattern as the cortical pyramidal (#298), Purkinje (#299),
FSI (#301), DA (#304), and STN (#305) presets: the Pospischil/MH92
Na⁺/K⁺ kinetics produce APs that overshoot a few mV past the literature
peak, with an AHP that runs a few mV deeper than reported. Likely
g_Na too high relative to leak, plus K⁺ deactivation slightly faster
than literature.

Acceptance criteria

When this issue is resolved, both xfail markers in
tests/integration/test_thalamic_relay.py should be removed and the
tests should pass. strict=True will fail the suite if either test
unexpectedly passes without the marker being removed.

References

• McCormick & Huguenard (1992) J. Neurophysiol. 68:1384 — TC AP shape