release: data-table-pro v5.1.0

[Dastari]

What changed

• Render the internal fill-layout spacer with the same header, body-row, and
  summary-row borders and backgrounds as adjacent data cells.
• Preserve the spacer's package-owned sizing, overflow, accessibility,
  keyboard-navigation, and export behavior.
• Add static, virtual, adapter, summary-row, and real-browser computed-style
  regressions.
• Document the behavior in the README, API reference, migration guide,
  roadmap, and changelog.
• Bump the package to 5.1.0 and commit the regenerated GitHub-installable
  distribution.

Why

The spacer explicitly forced border-b-0 and a transparent background.
Horizontal row and header borders therefore stopped at the last data column,
making the remaining width before right-pinned actions look like an unrelated
gap rather than an empty table segment.

Impact

This is a non-breaking visual fix. No consumer migration or configuration
change is required.

Validation

• frozen pnpm install and production dependency audit
• lint and package/demo typechecks
• 318 unit/integration tests with coverage
• package and demo production builds
• public API snapshot and bundle budgets
• packed v5.1.0 consumer typecheck/build
• Chromium fill/overflow layout regression