feat: merge atlas gr2 design stack (exec surface + prototypes)

[laynepenney]

Summary

• Merges Atlas's gr2 exec status surface (feat: add gr2 exec status surface #550) and prototype design stack (docs: stack shared scratchpad prototype on exec surface #556)
• Resolves doc conflicts from parallel merges (docs: ratify gr2 UX model and workflow #557, feat: add gr2 lane metadata surface #549, feat: add gr2 repo status surface #548, docs: design gr2 repo maintenance and lanes #543)
• Adds gr2 exec status command with lane-scoped execution planning
• Includes 7 Python prototype probes validating the lane model across 4 adversarial user modes

What's included

Rust (from #550):

• gr2/src/exec.rs: ExecStatusReport with load/render for lane-aware execution
• gr2/src/args.rs: ExecCommands subcommand
• gr2/src/dispatch.rs: Exec dispatch wiring
• gr2/src/lane.rs: Public load_lane() API
• tests/cli_tests.rs: 2 new integration tests (exec status + repo filtering)

Prototypes (from #556):

• cross_mode_lane_stress.py: 4 adversarial scenarios (solo, single-agent, multi-agent, mixed)
• concurrent_lease_stress.py: fcntl-based transaction locking (0% failure rate)
• lane_workspace_prototype.py: Full lane model with leases, conflict matrix, scratchpads
• real_git_lane_materialization.py + real_git_playground.py: Real-git verification
• cache_materialization_probe.py, layout_model_probe.py, repo_transport_probe.py

Docs:

• Updated manifesto, needs-and-criteria, shared scratchpads docs (latest iterations)

Conflict resolution

Doc conflicts resolved by taking the most complete version for each file:

• ASSESS-gr2-lanes-cross-mode-stress.md: kept main's version (257 lines vs 54)
• All others: took playground-stack's versions (later iterations, more complete)

Test plan

• cargo build passes
• cargo test - all unit + integration tests pass (761+ tests, 0 failures)
• New exec status tests included and passing

Premium boundary: core OSS (gr2 workspace orchestration).

Supersedes #550, #556. Close #554 as superseded.

[laynepenney]

Code review: Reviewed all Rust changes from #550 (exec.rs, args.rs, dispatch.rs, lane.rs, lib.rs, cli_tests.rs). Clean implementation with proper error handling, input validation, and comprehensive integration tests. No issues found. Prototypes from #556 verified via the design loop earlier today.