refactor(symfony-table): model rows/cells with enums instead of PhpMixed

The Table helper modeled a row's cells as PhpMixed and recovered the concrete
TableCell/TableSeparator types via runtime instance_of, leaving the entire
mixed/array bridge (to_row_vec, cell_colspan, row_get, ...) as todo!(). Replace
that with proper Row/Cell enums:

  Row  = HeaderDivider | Separator(TableSeparator) | Cells(Vec<Cell>)
  Cell = Null | Value(String) | Cell(TableCell) | Separator(TableSeparator)

The internal header/body boundary that PHP detects by object identity
($divider === $row) becomes the dedicated Row::HeaderDivider variant. Style
arguments (PHP string|TableStyle) become a StyleName enum, so Table::new no
longer panics in resolve_style's instance_of stub; TableStyle derives Clone so
named styles resolve from the registry.

PhpMixed-based callers (SymfonyStyle::table, render_table, auditor's sanitize)
bridge via From<PhpMixed> for Cell/Row at the boundary.

Un-ignores LicensesCommandTest text-format cases (4) and
BaseDependencyCommandTest::why; output matches PHP exactly. Removes ~15 impl
todo!()s in the Table helper.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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