PHASE_Z_B4_MAPPER_SOURCE env flag (default OFF) switches slot_payload
source-of-truth from legacy mapper-only / V4 rank-1 to B4 PlacementPlan
.selected_template_id at the single switch site in the runtime loop.
OFF preserves final.html SHA byte-equivalence (u4 parity guard, mdx 01-05).
ON requires Layer A render-active path; BLOCKED exits on B4 no-cover
and on B4-selected FitError (IMP-87 honesty gate pattern — NO silent
fallback). Distinct from PHASE_Z_B4_GATEKEEPER (mismatch render-skip).
Units (1 commit = 1 axis per Stage 1 scope_lock):
u1 — _b4_mapper_source_enabled() flag reader (default OFF)
u2 — _select_mapper_template_id() selector wired at the switch site
u3 — _b4_mapper_source_blocked_exit() for b4_no_cover / b4_selected_fit_error
u4 — render SHA parity regression (tests/regression/ baseline mdx 01-05)
u5 — slot_payload byte-equivalence (matches_mapper=True axis, mdx 01-05)
Targeted 89-a suite 63 PASS; Phase Z regression 323 PASS; IMP-87 mirror
20 PASS. Demo activation via .env only (no vite.config hardcoding).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
kyeongmin
b1bbe27c38