docs(planning): Branch E.1 stage0 self-host worksheet (bit-identical re-emit acceptance)

[briansrls]

What

W4-prep design worksheet for Branch E.1 stage0 self-host — the bit-identical re-emit acceptance shape. Authorized by PM (nimble-dove-733, 2026-06-03): hold E.1 impl, author this worksheet so the lane is ready the moment W4.7 dispatches and Rung 1 (#4353) lands.

Follow-on to RR-E (#4288) and E.3 DAG-regen design closure (#4302).

Scope

• Docs-only, one new file under docs/planning/. No .dag model, no Rust, no runner realization, no stage0 dissolution.
• Consumes, does not redeclare the already-modeled v4 self-host substrate: self_host.dag (FixedPointEqual/BootstrapRoundtripEqual/PromotionWitness, scaffolded fail-closed runner), the four-stage bootstrap.dag plan, and the three self-host TestClaims. Per SG-class discipline, this worksheet exists for the independent missing design fact — the acceptance shape that flips the declared runner from self_host_runner_not_realized to a real PromotionWitness — not to duplicate model facts.

Key content

• §10.0-adapted worksheet (migration class, representative failure, forbidden immediate-patch, DFS path E1.1–E1.6, deepest unsound boundary, systemic fix, non-goals, falsification probe, secondary metric).
• The bit-identical re-emit chain = SELF_HOSTING.md §7 meta-circular fixed point projected onto v4's seed → stage0 → stage1 → stage2 plan (digest(stage1) == digest(stage2) + canonical-source round-trip).
• Gate chain with current status: W3.2 coercion ✅ (W3.2 Leg B: refinement_widening rule + Widened + cross-target claims (PR) #4359), W3.4 spine 🟡, Rung 1 Execution-Runnable 🔴 OPEN (feat(runtime): gunbc test stage0 handler + corpus-eval CI host transport (Rung 1) #4353) — hard blocker, T-15 B1 content_hash 🔴, candidate generation 🔴.
• Forbidden gaming shapes named: placeholder-digest "fixed point", stage0 Rust cementing for a census ratchet, JSON-IR/emitted-Rust substitute for the H.7.2 canonical .dag source round-trip.

Why no impl

E.1 is W4.7, gated on runtime TestClaim execution: "self-host can't run until TestClaims actually execute at runtime" (PM). #4353 is still OPEN, so the runner correctly stays fail-closed.

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