branch_transfer: add coherence-witness diagnostics + collapse-constraint pipeline + IBM hardware runner

Author: christopher-altman

experiments/__init__.py

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+"""
+Experiment modules for ibm-qml-kernel.
+"""

experiments/branch_transfer/__init__.py

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+"""
+Branch-conditioned message transfer experiment.
+
+Implements the 5-qubit "partial branch-swap" protocol to test whether
+information written into a branch can be recovered after a partial reversal
+operation, providing operational constraints on decoherence/collapse models.
+
+Registers:
+    Q - measured qubit (initial superposition)
+    R - branch/room record
+    F - friend (observer proxy)
+    M - memory (message buffer)
+    P - paper (persistent record)
+
+Protocol:
+    1. Prepare Q in |+>
+    2. Correlate friend measurement: CNOT(Q -> F)
+    3. Record branch: CNOT(F -> R)
+    4. Write message (if mu=1): CNOT(F -> M)
+    5. Copy to paper: CNOT(M -> P)
+    6. Erase memory: CNOT(P -> M)
+    7. Partial branch-swap: X on (Q, R, F)
+    8. Measure (R, P) -> bitstring "PR"
+"""
+
+from .circuit import (
+    build_branch_transfer_circuit,
+    build_control_circuit,
+    build_coherence_witness_circuit,
+    compute_coherence_witness_from_counts,
+    compute_normalized_coherence,
+    compute_coherence_magnitude,
+    get_expected_ideal_coherence,
+    COHERENCE_QUBITS,
+)