Docs: drop PR-split L1a/L1b/L2/L5/L6 tags from distributed bring-up code

[ChaoWao]

Summary

PRs #608 / #610 / #613 used (L2) / (L5) / (L6) as split-step labels in their titles and commit messages. Those tags leaked into docstrings, comments, and the allreduce_distributed example where they collide with the framework hierarchy in docs/hierarchical_level_runtime.md — which also calls its levels L0–L6, but means CORE / DIE / CHIP / HOST / POD / SuperNode / Cluster.

A reader seeing L6 teardown order or L1a root-info handshake reaches for the topology definition and finds the two numberings disagree (framework L6 = cluster, split-step L6 = Worker bootstrap loop). PR #613's body already announced the cleanup for new and touched code; this PR finishes it on the files that were left behind.

• examples/workers/l3/allreduce_distributed/main.py — replace the fake "L1a..L6 stack" table with an unlabeled component list and a pointer to the real topology doc.
• tests/ut/py/test_worker/test_bootstrap_context_{sim,hw}.py — drop (L5) from module titles; rewrite L6 teardown order / L6's ChipContext / L5 one-shot bring-up / L1a root-info handshake / paired L1b UT to name the thing (Worker bootstrap loop, ChipContext, bring-up, etc.).
• tests/ut/py/test_worker/test_bootstrap_channel.py — (L2 bootstrap mailbox) → (per-chip bootstrap mailbox); the old tag was ambiguous with framework L2 (CHIP).
• tests/ut/py/test_worker/test_platform_comm.py — drop L1a prefixes in the HCCL backend references. The two L2-boundary contract sentences are left alone — those correctly refer to the framework L2 boundary documented in hierarchical_level_runtime.md.
• src/common/platform_comm/comm_sim.cpp — L1a contract alignment notes → HCCL backend contract alignment notes.
• src/a2a3/platform/onboard/host/device_runner.cpp — L1a 507018 → HCCL 507018; L1a C++ hardware UT → HCCL C++ hardware UT.

No runtime behavior change — comments and docstrings only.

Test plan

• grep -rnE '\bL1[ab]\b|\bL5\b|\bL6\b' across src/, python/, examples/, tests/ reports only the legitimate framework-hierarchy uses in docs/hierarchical_level_runtime.md and src/common/hierarchical/types.h.
• Pre-commit hooks (clang-format, clang-tidy, cpplint, ruff, pyright) all pass.
• No test run needed — zero code changes.

🤖 Generated with Claude Code

[gemini-code-assist]

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