Implement local harness and ABI runtime boundary

[epi13]

Summary

• Add bounded local-harness chat/start execution with timeout/output limits, identity-bound requests, diagnostic-only response normalization, and fail-closed authority checks.
• Add Git-validated detached proposal worktrees with source commit identities, root confinement, explicit preservation/cleanup, and append-only run records.
• Add the reviewed Rust v1 dynamic-library loader with SHA-256 artifact admission, descriptor/query/result validation, host-owned output copying, native cdylib fixtures, and existing-host quarantine integration.
• Add initial immutable binary transition, pair, and tabular diagnostic snapshots with dependency-bound identities.

Validation

• 46 Python unittest tests; compileall; Ruff.
• cargo fmt; clippy with -D warnings; 25 Rust tests/doc-tests.
• Editable install, learned-provider list, demo/ledger verification, provider benchmark.
• Real sibling local-harness health/check stdio smoke test.

The native loader remains native-trusted rather than an OS sandbox, and all investigator/provider outputs remain proposal-only or diagnostic-only.