Extract plural SCM compatibility realization shape

[gunbai-bot]

Summary

Extract the R1 compatibility-realization shape from the independently landed Git, Mercurial, and Pijul R0 models. The shared carrier contains only demonstrated presentation/capability roles: opaque target and generation handles, an independent fidelity disposition for every retained or lost axis, state-bearing projection/read-back identities, and modeled-or-unavailable conditional advance. Each adapter retains its upstream-native typed receipt.

This also removes the Git-shaped projection slot and Git-specific scenario names from the C0 native interaction contract. A submission carries realization receipts keyed by realization and target; a later independently read-back receipt replaces the prior row idempotently. Adding a fourth realization supplies a handler without editing the contract or introducing ScmKind/vendor dispatch.

Projection and read-back handles now fingerprint the actual observed repository state. The producer/version remains part of the read-back identity, but cannot collapse distinct Git, Mercurial, or Pijul observations. Capture facts have one authority: the per-axis fidelity profile. There is no separate exclusive capture flag that could contradict it.

R1 deliberately does not model semantic integration, policy admission, repair capability, executed advance, commit authority, or an Applied receipt. Executed advance and repair states wait for the later realization that actually produces those receipts.

Test plan

• C0 interaction contract, shared R1 shape, Git/Mercurial/Pijul handlers, full roadmap authority, and rendered-roadmap acceptance suite: 65/65 PASS
• One fixed C0 proposal binds the actual Git, Mercurial, and Pijul R0 fixtures through native decoders/read-back assessments and preserves proposal identity, transition, and primary outcome: PASS
• Three discriminating REDs hold producer/version and generation evidence fixed while changing independently observed repository state; projection and read-back identities change for Git, Mercurial, and Pijul: PASS
• Missing/differently grounded conditional advance and read-back mismatch refuse: PASS
• Synthetic fourth handler requires no primary-contract or kernel edit: PASS
• Canonical generated-artifact main_wet projection: PASS
• regen_stage0 --verify: PASS (regen_divergence_count=0)
• cargo fmt --all --check: PASS
• git diff --check: PASS