[6.x] Forms 2: Prefix internal submission keys with an underscore

[jasonvarga]

Submissions store their internal lifecycle metadata (partial, site) inline in their data. This renames those keys to _partial and _site so they're visibly internal and can't collide with form field handles. Neither key exists on a released version, so there's nothing to migrate.

• asPartial(), isPartial(), finalize(), site() and whereStatus() use the new keys. where('site', $handle) still works since it resolves through site().
• Adds Submission::values(), which returns the data without _-prefixed keys (_partial, _site, _fake). data() is untouched, so writes through it still persist.
• The AJAX form success response now returns values() rather than data(), so internal keys aren't exposed to visitors. Webhooks, exporters, emails and augmentation already only output blueprint fields.
• {{ form:submissions }} only skips its finalized-only default for status: conditions. The old partial: check no longer refers to anything, and Antlers drops _-prefixed params so _partial: isn't usable either. A user field named partial no longer disables the default.
• partial and site are now usable as form field handles. Handles starting with _ were already rejected by the Handle rule; this is now covered by a test.

values() isn't added to the Submission contract, as that would break direct implementers.

The eloquent driver will need to mirror the _partial/_site keys in whereStatus and its site queries.