Add efficiency operating hull to three-regime co-design study

[Copilot]

Extends the three-regime study with a mask-respecting efficiency operating hull (companion to the existing power hull), embeds both hull tables in FINDINGS_3REGIME.md, and adds divergence analysis showing the two co-design schedules differ at 17 of 27 period points.

Script changes (scripts/three_regime_comparison.py)

• _build_efficiency_envelope() — per-period max(η) hull with strict masking: excludes masked==True, linear_popt_invalid==True, NaN η, and η > 1+ε. All-masked periods emit eta_max=NaN, masked=True rather than fabricating a value.
• _write_efficiency_envelope_csv() — writes T_s, eta_max, controller, flap_angle, masked
• plot_operating_envelope_efficiency() — journal-style figure matching power envelope style; hatches all-masked periods
• main() — both hulls now regenerate in a single --plot-only invocation
• Added REGIME_LABEL_Y = 0.95 constant (applied to both envelope plots)

Documentation (analysis/FINDINGS_3REGIME.md §4)

Restructured §4 into three subsections:

• §4a — Power hull narrative + embedded 27-row table
• §4b — Efficiency hull with masking rule explanation + embedded 27-row table
• §4c — "Power vs efficiency co-design schedules diverge": the two hulls agree at T=1.5–2.25 s and T=2.75–4.0 s (resonance band), but diverge elsewhere due to two structural causes:
  1. Short T (≤1.25 s, 2.5 s): power hull favors VGM-0 for max raw excitation; efficiency hull picks the flap with the best P/P_opt ratio
  2. Long T (≥4.25 s): power hull uses opt_passive/VGM-0 even in the pitch-radiation notch (P_opt undefined, masked=true); efficiency hull correctly excludes these and selects ff+PID + resonance-matched higher-angle flaps

docs/CONTROLLERS.md

Added efficiency envelope outputs to the three-regime reproduction section with the mask-respecting rule documented.

.gitignore

Added allowlist exceptions for analysis/three_regime/*.csv and analysis/three_regime/figures/*.png so generated artifacts are committable.

Committed artifacts

• analysis/three_regime/operating_envelope.csv (power hull)
• analysis/three_regime/operating_envelope_efficiency.csv (efficiency hull — verified: no η>1 spikes)
• analysis/three_regime/figures/operating_envelope_efficiency.png