feat(ci): add test-telemetry workflow (nextest JUnit + slow tests)

[EffortlessSteven]

Summary

Adds a non-blocking Test Telemetry workflow that runs cargo-nextest on the CI Core crate set, emits JUnit XML via the existing .config/nextest.toml ci profile, and posts a slow-test summary to the run step summary.

This is PR M in the multi-PR CI roadmap.

Why a separate workflow

Switching the merge-gate test step (CI Core's build-test-linux) from cargo test to cargo nextest run changes test discovery semantics — notably, nextest doesn't run doctests. That belongs in its own deliberate PR. PR M ships the JUnit-shaped per-test data and slow-test visibility now without risking the merge gate.

What you get

• target/nextest/ci/junit.xml uploaded as nextest-junit artifact (30-day retention).
• Step summary with totals (tests / failures / skipped / wall time) and the 20 slowest tests (≥ 0.5s):

Test Telemetry

• Tests: 482
• Failures: 0
• Skipped: 18
• Wall time (sum): 47.3s

Slowest tests (≥ 0.5s, top 20)

Test	Time (s)
bitnet_quantization::i2s_qk256::round_trip_proptest	4.12
…	…

continue-on-error: true at the job level — failures here do not block merges. CI Core's build-test-linux remains the merge-gate source of truth.

Cost

~10–20 LEM per PR (one cargo nextest run over the CI Core crate set). Cached separately from ci-core-ubuntu so it does not contend with the gate.

Test plan

• Verify the workflow runs on this PR (it touches its own file).
• Verify nextest-junit artifact uploads.
• Verify the slow-test summary renders.
• Verify it is not in the required-checks list (advisory only).

Out of scope (future PRs)

• PR N: aggregate per-test duration history across runs into ci-actuals.json; consumes this PR's JUnit artifact.
• Switching CI Core's merge-gate test step to nextest (separate, larger PR; needs doctest plan first).

https://claude.ai/code/session_01S2yTnEYJcA3G2CyZn9bY4v

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