Bug 1941337: Sign the Delta column and group thousands in metric values

[sumairq]

Summary

Improves numeric formatting in the comparison results, addressing the two asks in
Bug 1941337:

1. Explicit sign on the Delta column — positive change percentages now render
   with a leading + (e.g. +53.31 % instead of 53.31 %), so direction is
   immediately readable alongside the existing - on negatives.
2. Thousands grouping on measurement values — large stat values now use a
   grouping separator (e.g. 2,113.69), completing the work started in Format numbers in revision and subtests revision rows #828 which
   covered the Base/New columns.

Locale stays hardcoded to en-US, per the decision recorded on the bug.

Changes

Sign (+) on directional percentages — via a small signPrefix helper:

• Student's-T Delta %: results table, subtests table, and the expanded
  "Difference of means" / "Difference of medians" lines.
• Mann-Whitney median-diff %: results table and subtests table.
• Left unsigned by design (not directional change values): CLES %, Cliff's Delta
  (−1…1), p-values, and confidence.

Thousands grouping on measurement values — via formatNumberFixedTwo (grouping

• fixed 2 decimals, matching the prior .toFixed(2) appearance):

• Mann-Whitney expanded metrics table: mean, median, variance, std dev, min, max.
• RunValues: the overflow individual run values and the standard-deviation line
  now group consistently with the first-100 values and Base/New columns from Format numbers in revision and subtests revision rows #828.

Notes / decisions

• Percentages are never grouped — only measurement values are. A Delta % gets a
  sign, not a separator.
• Decimal places are preserved where they mattered. The metrics cells keep
  exactly 2 decimals (mirroring .toFixed(2)); RunValues reuses the existing
  formatNumber, so its values follow the same ≤3-decimal display as the rest of
  the column (real run values are ≤2 decimals, so this is not visible in practice;
  full precision is still preserved by the "Copy results" button).
• Formatters are module-level cached instances, consistent with the existing
  formatNumber / dateFormatter in src/utils/format.ts.

🤖 Generated with Claude Code

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