Issue #3605421: Show a process instance variable as a typed Views column

Adds a configurable Process variable Views field on the process instance, so a dashboard can show a named process variable as its own column.

Configure the field with:

• Variable: the variable to show. A dotted path reads into a structured value, the same syntax flow conditions use: order.total shows the total key of the array held in order. A plain name shows a scalar.
• Display as: Text, Number, Date (a Unix timestamp or a date string, formatted with the chosen date format), or Yes/No for a boolean.

The field reads the instance-wide value, never a branch token-local one, and the named variable is batch-loaded for the whole page in a single query, so a column over many instances does not read per row. Add the field more than once to show several variables side by side.

What is included

• New Views field handler orchestra_instance_variable and its declaration in views_data_alter.
• Config schema for the handler options.
• A kernel test covering the dotted path, every display type, the missing key fallback, instance-wide vs token-local reads, and the per-page batch load.
• Docs: a "Process variables as columns" section in the Views guide.

phpcs (Drupal, DrupalPractice), phpstan (level 5) and cspell are clean; the new and the extended Views tests pass.