Retire Classic UI and unify flamegraph/API contracts

[reiase]

Summary

• Retire the Classic web shell (app.rs / pages/ / ui_version); main.rs mounts Next only, with dead Classic helpers and unused imports cleaned so WASM builds warn-free.
• Move distributed flamegraphs behind extension SSOT (/apis/pprofextension/.../distributed/json, /apis/torchextension/.../distributed/json); remove training public aliases and stop server/training.rs from calling probing_python::features::*.
• Point the Web PyTorch profile client at /apis/pythonext/pytorch/profile/start; mark removed paths in api_spec.json deprecated_paths with status: removed.
• Align modularity/contributing/skills docs with reality (cdylib cli_main in root src/lib.rs; content SSOT at python/probing/bundled_skills/); stub architecture.zh.md; archive root REVIEW*.md.

Breaking changes

• Removed public routes: /apis/training/distributed_flamegraph/json, /apis/training/distributed_stack_flamegraph/json (use extension paths above).
• Classic UI URLs / Classic↔Next switch are gone; unknown routes fall through to Next Explore.

Test plan

• cd web && cargo check --target wasm32-unknown-unknown (expect 0 warnings)
• cd web && cargo test --bins (120 tests)
• pytest tests/regression/spec/ tests/regression/profiling/test_distributed_flamegraph.py -q
• cargo test -p probing-rust-regression --test server_training_observability --test python_distributed_flamegraph
• cd web && dx bundle --release --debug-symbols false
• Smoke: Next shell loads; Stacks → Distributed; ⌘K eval; Investigate pending actions from source bridge

Made with Cursor

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
...egression/profiling/test_distributed_flamegraph.py	94.04% <100.00%> (+0.14%)	⬆️

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