refactor(duplex-metrics): replace dead Binomial::new Err arm with expect

[nh13]

Summary

In calculate_ideal_duplex_fraction, the match Binomial::new(0.5, size as u64) { Err(_) => continue, … } arm is statically unreachable — statrs::distribution::Binomial::new(p, n) only returns Err(BinomialError::ProbabilityInvalid) when p is NaN or outside [0, 1] (see statrs-0.18.0/src/distribution/binomial.rs:67-70), and every call site here passes the literal 0.5.

Replace the match with .expect(\"…\") so that any future refactor letting p become dynamic surfaces the panic immediately instead of silently dropping families from the ideal-fraction calculation.

Diff

One file, 4 lines net:

- let binomial = match Binomial::new(0.5, size as u64) {
-     Ok(b) => b,
-     Err(_) => continue, // Skip if binomial creation fails
- };
+ // `Binomial::new(p, n)` only returns `Err` when `p` is NaN or
+ // outside `[0, 1]` (statrs 0.18 `BinomialError::ProbabilityInvalid`).
+ // With `p = 0.5` hardcoded the `Err` arm is statically unreachable;
+ // expect rather than silently skip so a future refactor that lets
+ // `p` become dynamic surfaces immediately instead of silently
+ // dropping families from the ideal-fraction calculation.
+ let binomial = Binomial::new(0.5, size as u64)
+     .expect(\"p = 0.5 is always a valid probability for Binomial::new\");

Behaviour

None — the replaced arm could not have fired on any input. Spotted during review of #359; sized as a separate PR per reviewer guidance to keep that PR's diff focused on the OOM fix.

Test plan

• `cargo test --release --lib duplex_metrics` — 20 passed
• `cargo ci-fmt`, `cargo ci-lint` clean

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 4ba90d59-d4cf-4f0e-a5c3-ff19370e59d4

📥 Commits

Reviewing files that changed from the base of the PR and between cffd59b and 6c98524.

📒 Files selected for processing (1)

• src/lib/commands/duplex_metrics.rs

---

📝 Walkthrough

Walkthrough

In calculate_ideal_duplex_fraction_per_size, Binomial::new(0.5, size as u64) was changed from a match that skipped families on Err to an expect that panics on Err. The code now assumes Binomial::new cannot fail and will abort if it does.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	Title accurately summarizes the main change: replacing an unreachable error branch with expect() in the duplex metrics code.
Description check	✅ Passed	Description clearly explains the change, provides context about why the Err arm is unreachable, and documents the behavioral impact.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch nh/drop-dead-binomial-err-arm

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.00%. Comparing base (c3b6dc6) to head (6c98524).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #363      +/-   ##
==========================================
+ Coverage   90.93%   91.00%   +0.06%     
==========================================
  Files          78       78              
  Lines       51046    51045       -1     
==========================================
+ Hits        46421    46451      +30     
+ Misses       4625     4594      -31     

☔ View full report in Codecov by Sentry.
📢 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.

[nh13]

@coderabbitai review

[coderabbitai]

✅ Actions performed

Review triggered.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.

[nh13]

@coderabbitai review

[coderabbitai]

✅ Actions performed

Review triggered.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.

[nh13]

@coderabbitai review

[coderabbitai]

✅ Actions performed

Review triggered.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.

[nh13]

@coderabbitai review

[coderabbitai]

✅ Actions performed

Review triggered.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.

[nh13]

@coderabbitai review

[coderabbitai]

✅ Actions performed

Review triggered.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.