Fix partially empty pipe table separator regression

[vicancy]

Summary

Restore Markdig's historical support for individual empty or whitespace-only pipe-table separator cells, while retaining rejection of separator rows with no dashes.

Commit 7959e3b fixed #927 by rejecting pipe-only separator rows. It also changed per-cell validation to reject empty cells, so previously accepted tables such as the following now render as literal paragraphs:

| Field | PersonShared | Person |
| --- | | --- |
| Name | Master | Inherit |

This regression was reported in dotnet/docfx#11147 after its Markdig upgrade. The reporter's follow-up distinguishes a partially empty separator from the entirely pipe-only rows targeted by #927.

This is a backward-compatibility fix, not a strict GFM compliance change: strict GFM requires dashes in each separator cell.

Changes

• Allow empty/whitespace-only cells in TryParseHeaderColumn, with no explicit alignment and zero dash count.
• Keep FindHeaderRow's totalDelimiterCount == 0 guard unchanged, preserving the all-pipe-row fix and preventing division by zero when inferring widths. Keep trailing-pipe handling unchanged.
• Add canonical HTML specs and regenerate their tests for the reported table, first/middle/last empty cells, adjacent pipes, alignment, optional outer pipes, header-only tables, and whitespace-only separator rows.
• Add inferred-width cases and rejection checks with width inference on/off and with/without a body. Invalid nonempty cells still reject the table.
• Document the compatibility behavior and zero-width contribution of empty separator cells.

Validation

• Before the parser fix: the selected .NET 10 table/emoji run had 141 passing tests and 14 expected failures: six new HTML specs (including the reported reproduction) and eight new inferred-width cases.
• After the fix: all 155 selected table/emoji tests passed on each of .NET 8 and .NET 10.
• Full Markdig test suite: 3,834 passed, 0 failed, 1 existing ignored test (ListUnorderedLooseTop) on each of .NET 8 and .NET 10.
• Release library build: succeeded for net462, netstandard2.0, netstandard2.1, net8.0, and net10.0, with 0 warnings and 0 errors.

Commands run from the repository root:

dotnet test .\src\Markdig.Tests\Markdig.Tests.csproj -c Release -f net10.0 --no-restore --filter "FullyQualifiedName~PipeTable|FullyQualifiedName~Emoji"
dotnet test .\src\Markdig.Tests\Markdig.Tests.csproj -c Release --no-restore --filter "FullyQualifiedName~PipeTable|FullyQualifiedName~Emoji"
dotnet build .\src\Markdig\Markdig.csproj -c Release --no-restore
dotnet test .\src\Markdig.Tests\Markdig.Tests.csproj -c Release --no-restore

Validation was on Windows; the documentation site and unrelated solution tools/benchmarks were not built. No dependency or SDK policy changes.