CODAP-1471: omit LSRL statistics that aren't estimable

[kswenson]

Fixes CODAP-1471

⚠️ Stacked on #2674 (CODAP-1461). Targets that branch, not main — retarget
once #2674 merges. Review only the latest commit; the rest is #2674's diff.

Problem

Three cases where a statistic that can't be estimated still reached the LSRL's output:

Data	Before	After
Horizontally aligned + Show r/r²	y = 5, r = (not a number), r² = 0	y = 5
Two points + Confidence Bands	σslope = 0, σintercept = 0, NaN band paths, 3 console errors	equation only, paths cleared, no errors
Normal data	r, r², σ, bands	unchanged

Root causes

• rSquared is (sumOfProductDiffs)² / (xSumSquaredDeviations * ySumSquaredDeviations),
  which is 0/0 when the y values don't vary. The r branch formatted that NaN directly
  while the r² branch passed it through rSquared || 0 — hence the two disagreeing.
• linRegrStdErrSlopeAndIntercept special-cased count === 2 to return {0, 0}. With
  zero degrees of freedom the standard errors aren't estimable, so 0 asserts something
  we can't know.
• mse = sumSquaredErrors / (count - 2) is 0/0 with two points, and confidenceValues
  guarded with == null, which NaN passes. NaN reached the band path d attribute, so
  the browser rejected the paths and logged <path> attribute d: Expected number.

Fix

Per the rule agreed with Bill: anything undefined is omitted — geometry that can't be
drawn isn't drawn, numbers that can't be computed get no row.

• lsrlEquationString omits the r, r², σslope and σintercept rows unless the value is
  finite, and no longer launders NaN through || 0.
• linRegrStdErrSlopeAndIntercept drops the count === 2 special case.
• confidenceValues guards on finiteness rather than nullness, and the component clears
  the band paths instead of assigning empty/NaN ones.

V2 compatibility

I ran v2's own math in the live v2 app (build 0745). v2 produces the same NaNs for every
one of these cases and has no count === 2 → 0 special case — it returns NaN for
seSlope/seIntercept with two points. So removing that special case restores v2 behavior
rather than diverging from it. On 5-point data v3 now yields σslope = 0.15 and
σintercept = 0.497, matching v2's 0.15000000000000005 / 0.49749371855331026.

Testing

• New lsrlEquationString tests: r/r² omitted when rSquared is NaN, σ rows omitted when
  the standard errors are NaN, plus a positive control that finite values still render.
• New confidenceValues / confidenceBandsPoints tests: no bounds and no band points
  with two points; finite bounds with four.
• Updated the two-point linRegrStdErrSlopeAndIntercept expectation from 0 to NaN.
• Verified in the browser for all three rows of the table above, including that the
  console errors are gone.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.69%. Comparing base (739dee8) to head (b06718f).

Additional details and impacted files

@@                         Coverage Diff                          @@
##           CODAP-1461-lsrl-degenerate-equation    #2675   +/-   ##
====================================================================
  Coverage                                87.69%   87.69%           
====================================================================
  Files                                      810      810           
  Lines                                    46938    46939    +1     
  Branches                                 12013    12004    -9     
====================================================================
+ Hits                                     41162    41164    +2     
+ Misses                                    5760     5759    -1     
  Partials                                    16       16           

Flag	Coverage Δ
cypress	69.50% <100.00%> (+<0.01%)	⬆️
jest	62.71% <70.00%> (+0.04%)	⬆️

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[cypress]

codap-v3    Run #12102

Run Properties:   Passed #12102  •  e2bd79a758: null

Project	codap-v3
Branch Review	main
Run status	Passed #12102
Run duration	07m 54s
Commit	e2bd79a758: null
Committer	null
View all properties for this run ↗︎

---

Test results
Failures	0
Flaky	1
Pending	82
Skipped	0
Passing	382
View all changes introduced in this branch ↗︎

[Copilot]

Pull request overview

This PR updates the v3 graph LSRL (least-squares regression line) display logic so that regression-related statistics and confidence-band geometry that are not mathematically estimable (e.g., NaN from degenerate inputs like two points or no variance) are omitted/cleared rather than rendered, preventing inconsistent UI output and browser console errors.

Changes:

• Leave LSRL standard errors as NaN when degrees of freedom are zero (two-point fit), and update unit tests accordingly.
• Update lsrlEquationString() to omit r, r², and σ rows unless the computed values are finite (and stop laundering NaN through || 0).
• Guard confidence-band computation/rendering against non-finite inputs and clear SVG paths when bands can’t be estimated.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
v3/src/utilities/stats-utils.ts	Stops reporting 0 standard errors for 2-point fits; leaves them non-estimable (NaN).
v3/src/utilities/stats-utils.test.ts	Updates expectations to match non-estimable standard errors for 2-point data.
v3/src/components/graph/utilities/graph-utils.ts	Omits non-finite LSRL statistics from the equation string (r, r², σ terms).
v3/src/components/graph/utilities/graph-utils.test.ts	Adds tests asserting omitted/retained rows based on finiteness of stats.
v3/src/components/graph/adornments/lsrl/lsrl-adornment-model.ts	Switches confidence-band bounds gating from nullness to finiteness to avoid NaN propagation.
v3/src/components/graph/adornments/lsrl/lsrl-adornment-model.test.ts	Adds tests covering confidence bounds/band points behavior with sufficient vs insufficient degrees of freedom.
v3/src/components/graph/adornments/lsrl/lsrl-adornment-component.tsx	Clears band paths when unestimable (returns null paths; avoids empty/NaN path strings).

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[bfinzer]

👍🏻LGTM