PR 3/3: Crucible backend (capstone)

[ericeil]

PR 3 of 3 — Crucible backend (capstone)

Part of the stacked split of eric/crucible (see docs/pr-split-plan.md).
Stack: master → eric/ecosystem → eric/rust → eric/crucible-app.
Base: eric/rust (PR #97) — review PRs #96 and #97 first.

The tip of the stack — the Solana verification application, wiring PR 1 (ecosystem) +
PR 2 (rust framework) + the upstream command sandbox together.

What this adds

• composer/crucible/* and rust/crucible-app (the Crucible wheel)
• Crucible RAG: committed manifest rust/crucible-app/crucible_kb.rag.json + shared
  composer/scripts/rag_import.py, composer/rag/{import_format,db}.py, composer/tools/crucible_rag.py
• ReportBackend "crucible" + render labels + as_report_backend
• Sandbox default → launcher for crucible (fail-closed; the launcher itself is upstream)
• test_scenarios/solana_vault + the crucible test gates — the vault sample also makes PR 2's
  test_solana_gate runnable from here
• docs/application-abstraction.md — the five pieces of an analyzed application / Rust app
  framework (moved here from the ecosystem PR, where it did not belong), plus the crucible-*
  design docs

Finalizes the PR 2 cross-cutting intermediates

• rust/Cargo.toml re-adds the crucible-app workspace member.
• rustapp/adapter.py swaps the tag cast back to the validating as_report_backend, now that
  report/schema.py closes ReportBackend to {prover, foundry, crucible}.
• Carries the crucible-specific sandbox tweaks: sandbox/recipes.py per-run RUSTUP_HOME;
  docker-compose.sandbox.yml un-gated run-confined-build.

Verification

• cargo build (all 4 crates) ✓
• crucible-layer modules import ✓

Notes

• Draft: the real-LLM gates were not run here — test_crucible_gate,
  test_crucible_setup_gate, test_crucible_formalize_gate, test_crucible_e2e_gate.
  (setup/formalize were verified green earlier this cycle against a real crucible build.)

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