M4b-1: the spin track foundation — the DC control class, the charge-landscape family, the sensor-response path

[aarontrowbridge]

Closes #35

The spin track stands up: a third channel of control joins the contract.

• The DC control class: set_gate!/get_gate/gate_snapshot verbs in BASE (the waveform verbs sibling; unknown gates named with the known list) + the DCAxis/DCAxisMap instrument data surface; TwinSoc gains a gates CONTROL field — the three-way separation asserted live (drift moves truth only, gates are controls, calibration moves belief only); the pulse path refuses a landscape family actionably — two control classes, one twin
• The dc.py bridge: StrumentoDCSource (PythonCall ext) delegating 1:1 onto the landed strumento.core.dc DCSource seam (MockVoltageSource default; Python owns clamping/addressing; set_compensated deliberately out of v1 — compensated moves are procedure-layer); python-optional items skip cleanly
• The spin family: the vendored constant-interaction double-dot over (2,0)/(1,1)/(0,2) — closed-form eigen-structure verified against the direct eigensolve to 1e-15, the sensor chain (ground-state left-dot occupancy, quasi-static charge-noise rounding, then the record PSB confusion + binomial shots through the ONE response home); scale conventions documented honestly (2t_c = J_max identification; the lever-arm placeholder cannot mask physics changes — all pins are ratios/diagonal positions)
• The sensor-response path: charge_sensor_sweep (exact + sampled) — monotone step between confused plateaus, crossing its midpoint exactly on the gate diagonal (|v*| < 0.02 V) for every common mode; truth-dependence seeded pins (J_max x2 -> width x1.73, delta x3 -> x1.86, bands asserted); drift through advance! + rebuild moves the flanks
• Replay: fresh-process diff-empty; the fixture lane runs the chain Python-free (897/897); the bridge lane 961/961; base-only HOLDS 301

ESCALATION (pre-existing, proven on pristine main, CI unaffected): the pythoncall-only local lane errors on three keystone/Rabi items that use JSON without the weakdep in that manifest — a chore-tier follow-up.

Verification (director-run): 961/961 bridge-exercised (12m41s); base-only HOLDS 301.