fix(vector): VECTOR(N) columns round-trip, and the case-exact doc-link guard that greens the ubuntu CI leg - #448

What this lands

1. VECTOR(N) columns round-trip (session 85). The DataType.Vector case was missing from
eight copies of the per-type row codec, five value tokenizers, the single-file JSON reader and both
parameter binders, so a float[] was persisted as the literal text "System.Single[]". Full
breakdown in docs/CHANGELOG.md → [Unreleased] → Fixed.

2. The red ubuntu leg of CI/CD Pipeline on master is fixed here (session 85, addendum 2).

master at 204d9f8a — this branch's own merge — failed Build and Test (ubuntu-latest):

DocumentationConsistencyTests.DocumentationMarkdownLinks_ShouldResolveToExistingFiles [FAIL]
  Assert.Empty() Failure: Collection was not empty
  Collection: ["Vectors/VECTOR_MIGRATION_GUIDE.md:793 -> ../vector...",
               "Vectors/VECTOR_MIGRATION_GUIDE.md:794 -> ../vector..."]

Two layers, both fixed:

• The links. docs/Vectors/VECTOR_MIGRATION_GUIDE.md pointed its resource links at
  ../vectors/README.md while the directory is docs/Vectors — fine on Windows and macOS, 404 on a
  case-sensitive host. They now read ../Vectors/README.md, the form every other vector link in
  docs/ already used.
• The guard that should have caught this on any host. File.Exists/Directory.Exists are
  case-insensitive on Windows and macOS, so the link test added by this branch's own documentation
  audit (bdb4e094; absent at 92794084, the last green master) could not fail on the windows or
  macos leg — it failed only on ubuntu, after the merge. Both copies of that resolution (the link
  sweep and DocumentationIndex_ShouldOnlyReferenceExistingDocumentationFiles) now go through one
  PathExistsWithExactCase helper that matches every path segment ordinally, so a wrong-case link
  fails locally too.

Coverage (net11.0) was collateral, not a second defect: tools/measure-coverage.ps1 exited 2 on
that same single failure (FAILED SUITES: SharpCoreDB.Tests - coverage verdict withheld), so the
18 % floor was never evaluated. No other job failed — windows and macos were green, and
Compatibility Smoke Tests passed.

Verification

Red → green on Windows, which is the point of the guard change:

Build: 0 errors. CI's own filter everywhere: Category!=Debug&Category!=Manual&Category!=Performance.

And on the runners, which is the verdict that counts here — CI/CD Pipeline run 38163780903 (head
62908c7a) completed with every job success: Build and Test on windows-latest, ubuntu-latest and
macos-latest, plus Coverage (net11.0). Compatibility Smoke Tests succeeded as well. The coverage job
no longer exits 2 with a withheld verdict; it evaluates the floor again:

TOTAL    60529 lines   27819 covered   45.96%
PASS  measured 45.96%, floor 18.00%

This PR is left open and green for the owner to merge.

Merge notes

• A green push to master runs Pack NuGet Packages and Publish to NuGet.org (verified on the
  previous green master push, run 38130514269, where both succeeded). This PR does not merge itself
  for exactly that reason — merging is the owner's action.
• The two BLOCKED dependency items (Dependabot's 15 npm alerts in the server-script client's
  package-lock.json, and the transitive SixLabors.ImageSharp 3.1.11 advisory in the Orchard Core
  example) remain outside this PR: they need a package-version decision, not a code change.
• Worklog: docs/performance/WORKLOG.md, session 85 and its two addenda.