Stabilize normalized form-submission context for extensions

[danieliser]

Outcome

Provide the smallest Core contract Pro needs to identify and enrich a normalized form submission without importing storage, token, subscriber, or Lead Magnet concepts into Core.

Tracks the cleanup and completion of existing draft PR #1292. Cross-repository parent: https://github.com/PopupMaker/Pro/issues/132

Scope

• Add a stable submission identifier.
• Add source post/page resolution.
• Allow extension-owned context data.
• Expose the JavaScript submission-arguments filter for supported frontend flows.
• Populate provider and native submission identifiers where the provider exposes them.
• Document the normalized event shape and compatibility rules.
• Remove product-specific terminology from the Core contract.

Out of scope

• Field schema discovery.
• Dynamic-value registration.
• Submission persistence.
• Subscriber/profile collation.
• Actions or CRM mappings.
• Lead Magnet behavior.
• Product-tier decisions.

Acceptance criteria

• Existing integrations retain their provider keys and success behavior.
• The normalized payload supports submission_id, source_post_id, source URL, provider/form identifiers, optional native entry ID, popup/session context, and extension-owned context.
• Context can be supplied without changing conversion attribution.
• PHP and JavaScript tests cover the contract and documented extension points.
• PR Add normalized form submission context #1292 is cleaned into a reviewable history or replaced by a clean branch/PR with equivalent scope.
• No content_upgrade or lead_magnet contract is introduced in Core.

Expected PR

One Core PR, using existing #1292 if its history can be cleaned safely.