session_viability_v4: fresh-init parity harness for v4

3-mode subset (D_full_history, A_ams_prefix, C_ams_hybrid) on the same
10-query synthetic session as PR #29's session_viability.py, but using
MemLLM4 for A and C.

Fresh-init expectation: A/C hit-rates at Qwen2.5-1.5B scale on GPU are
not expected to beat v3.46 fresh-init — that requires training (v4.6).
This harness produces the fresh-init baseline that v4-trained will be
compared against.

B modes (B_flat_cos, B_ams_text) omitted: they are RAG-shaped upper-bound
diagnostics, not v4 product modes (per SPRINT_CLOSEOUT_v3.46.md §10.9).
D_full_history is kept as the ceiling baseline.

Co-authored-by: FluffyAIcode <FluffyAIcode@users.noreply.github.com>

Author: cursoragent