feat(export): add point-in-time projections

[tcballard]

Summary

• adds decided export --at <revision> for viewer, documents, and graph projections
• reconstructs historical governing config and recursive federation/submodule closure from exact committed blobs using read-only, offline Git object access
• preserves requested-path identity and existing public Rust construction contracts while failing closed on missing objects, unsafe paths, and bounded snapshot limits
• prevents snapshot writes inside the worktree, Git directory, or Git common directory

Verification

• cargo clippy -p asdecided-core -p decided --all-targets -- -D warnings
• 31 focused point-in-time tests: 10 engine snapshot tests and 21 CLI/federation/guard tests
• affected existing CLI, export, and federation regression suites
• release build: cargo build --release --locked -p decided
• corpus validation: 469/469 artifacts valid; relationships, review, and gate completed without blockers
• clean committed-tree byte parity for viewer, documents, and graph projections
• SurfaceCheck: 42 passed, 0 failed

A monolithic cargo test --workspace --release attempt exhausted the shared volume while LTO-linking integration binaries; it reported no code or test failure before storage exhaustion. The focused suites and release binary build completed successfully.

Closes #257