docs: repair The monolith section against the current per-chunk streaming architecture

[WilfordGrimley]

Summary

1. docs/features/stage-e-operations.md's "The monolith" section described run_pipeline's original 2026-07-30 shape (pooled Stage C subprocess via run_image_evidence_cohort, then a separate bulk Stage D pass). PR Streaming pipeline default: micro-batch Stage C/D dispatch and verdict transfer #666 (2026-07-31) replaced this with a per-chunk dispatch_micro_batch streaming loop, but only added a new proposal doc — this existing living reference was never updated. Corrected the pipeline diagram and added a dated note pointing out that _run_stage_c/_run_stage_d_bulk/the run_image_evidence_cohort delegation this section still describes in detail further down are dead code as of this reading (not reachable from handle() under any flag).
2. Same note records a second, verified finding: the per-chunk loop is still gated by settings.STAGE_E_STREAMING_ENABLED, which still defaults to False in both settings.py and docker-compose.prod.yml. dispatch_micro_batch checks that flag unconditionally for every caller, including run_pipeline — so with the flag at its default, every chunk the monolith dispatches reports status="disabled" and Stage C/D perform no work at all, while Stage 0/E preflight/channel_report still run and the command exits 0. This contradicts run_pipeline's own module docstring ("a complete, working, from-scratch, whole-catalogue run THAT WRITES... None is a precondition").

Test plan

• python3 .github/scripts/docs_lint.py --strict — clean
• pre-commit run --files docs/features/stage-e-operations.md — prettier, EOF, trailing-whitespace, readme-parity all passed
• Every claim in the added note traced to code, not the PR descriptions that reported it: grep -rn STAGE_E_STREAMING_ENABLED across cardpicker/, settings.py (default=False), docker-compose.prod.yml (${STAGE_E_STREAMING_ENABLED:-False}), dispatch_micro_batch's own gate check, and run_pipeline.py's handle() (confirmed _run_stage_c/_run_stage_d_bulk have zero call sites, _run_streaming_stages is the only path taken)