Add Qwen3 component harness: torch reference, equivalence helper, microbench helper

[chhwang]

Add Qwen3 component harness: torch reference, equivalence helper, microbench helper

Shared scaffolding under examples/qwen3/ for Q4–Q8 component work.

Torch reference (qwen3_ref.py): pure-torch Qwen3-8B forward —
RMSNorm, RoPE (θ=1e6), QK-norm, GQA (32q/8kv, head_dim 128), SwiGLU MLP
(intermediate 12288), TransformerBlock, full model. Random-weight init with
fixed seed, fp16. No ARK dependency.

Config (qwen3_config.py): parameterized Qwen3Config dataclass with
8B defaults. All fields overridable for future 32B stretch.

Equivalence helper (equiv.py): assert_close() wrapping
torch.testing.assert_close with shape, max abs/rel error, and per-tensor
statistics on mismatch.

Microbench helper (microbench.py): L2-flush buffer (128 MB),
CUDA-graph capture, pilot-driven iteration tuning, torch.profiler timing.
Returns {"mean_us", "std_us", "n_iters"}.

Tests (test_harness.py): 9 tests, all gated with
@skipif(not torch.cuda.is_available()). Coverage: reference forward shape,
RMSNorm unit-RMS check, causal mask validation, RoPE non-identity,
equivalence pass/fail/shape-mismatch, microbench eager + CUDA-graph.

Files are non-overlapping with Q1 (bench/) and Q2 (PROFILE.md).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.70%. Comparing base (c257202) to head (32261fc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #265   +/-   ##
=======================================
  Coverage   85.70%   85.70%           
=======================================
  Files         129      129           
  Lines        6457     6457           
=======================================
  Hits         5534     5534           
  Misses        923      923           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[chhwang]

Closing without merging. The harness code (torch reference, equivalence + microbench helpers under examples/qwen3/) is retained on branch qwen3-q3-harness and carried by the component PRs that build on it; not landing it as a standalone PR.