Add normalized form submission context

[danieliser]

Summary

• extend Popup Maker's normalized PHP and JavaScript form-submission contracts with provider submission IDs, source post IDs, and extension-owned context
• add pum.integration.form.submissionArgs so extensions can enrich every provider through one frontend integration point
• populate the new server-side identifiers for Fluent Forms without coupling Core to Content Upgrades
• document and test the backward-compatible defaults and extension context flow

Why

This is the small Core boundary needed by PopupMaker/Pro#127. Core remains unaware of Incentives, Assets, and Delivery; Pro consumes the normalized submission metadata.

Validation

• focused PHPCS: 0 errors (the existing integration registry reports its pre-existing alignment warnings)
• changed JavaScript file lint: pass (the already-touched legacy file was mechanically updated to current syntax so changed-file CI can enforce the repository rules)
• normalized PHP context exercised successfully in the linked Pro wp-env
• regression tests added in FormSubmissionContext_Test.php

Linked work

• PopupMaker/Pro#129 depends on this PR

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[danieliser]

Tracking issue: #1357. This draft should remain the minimal, product-neutral normalized submission-context seam for the Pro forms platform epic PopupMaker/Pro#132. Product-specific tokens, storage, mappings, actions, subscribers, and Lead Magnet behavior are out of scope. If the merge-heavy history cannot be cleaned safely, replace this PR from a fresh feature branch while preserving the tested contract.

[danieliser]

Superseded by clean draft #1359. The replacement preserves the product-neutral contract, adds source URL and focused JavaScript coverage, and avoids this branch's merge-heavy history.