fix(audit): keep campaign history in write order within the same second

[TheAggressive]

Closes #164. Part of #65.

Problem

Audit events store whole-second created_at values and random IDs, and history was ordered created_at DESC, id ASC. Events written in the same second therefore showed in arbitrary order. For example, a scripted create → snapshot → submit → approve listed "Approved" below "Created".

Change

• Schema 6 adds sequence_number to {prefix}campaignbridge_audit_events. It's a unique AUTO_INCREMENT insertion order.
• History is now ordered created_at DESC, sequence_number DESC. The target_created index still serves the filter.
• Migration: dbDelta adds a column and its index in separate statements, and MySQL refuses an AUTO_INCREMENT column that isn't a key. So Schema_Manager adds the column and its key in one ALTER before dbDelta runs, and dbDelta then sees nothing to change. Fresh installs get the column from the CREATE TABLE.
• Existing events are numbered in the order the database reads them, because their order within a second was never recorded. No existing value changes.
• Docs:
  • api.md: the history order.
  • campaign-persistence.md: the new column.
  • runbook.md: how to roll back to a schema-5 release. That code never reads the column, so lowering the stamped version is safe.

Evidence

• Migration from schema 5 on a populated table (Campaign_Schema_Migration_Test):
  • existing events get distinct, non-zero numbers;
  • a re-run changes nothing;
  • resuming after an interruption (column present, version still 5) changes nothing;
  • the next insert numbers above the existing rows;
  • the index list includes sequence_number.
• Same-second ordering (Campaign_Operation_Repository_Test): four events in one second, with IDs that sort against write order, read back newest write first, including across pages. A later second still sorts first even when written earlier.
  • Restoring the old id ASC ordering makes this test fail.
• SQLite (WordPress Studio): the migration on the local site kept all 281 audit events. Each got its own number, the column is auto_increment, and a second run was a no-op.

Verification

• pnpm qa:fast passes.
• The unit (1126), integration (431), security (117), accessibility (25), and performance (7) suites all pass.

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