fix(comparator): fix change detection for schema evolution and cold starts

[jcscottiii]

This change fixes a bug where new browser implementation data was not triggering notifications if the feature previously had no browser data (the "Cold Start" problem). It also standardizes how optional fields are compared to support clean schema evolution.

Key Changes:

• Comparator Logic (lib/blobtypes/v1/comparator.go):

  • Refactored compareBrowserImpls to remove an early return when the old state is unset. It now correctly detects when browser data is added for the first time.
  • Standardized all compareXYZ methods to explicitly handle the four states of OptionallySet fields: Unset/Unset (No Change), Unset/Set (Added), Set/Unset (Removed), and Set/Set (Value comparison).
  • Implemented "Quiet Rollout" logic for browsers: additions are ignored if the status is "Unavailable" and no other details (version/date) are present, reducing noise during backfills.
  • Refactored Name comparison to be robust against Unset states.

• Documentation:

  • Added a "Guide for Adding New Fields" to comparator.go to ensure future maintainers follow the same robust comparison patterns.
  • Documented the "Quiet Rollout" and "Cold Start" philosophies.

• Testing:

  • Added test cases to verify parent-level container additions (e.g., adding the first BrowserImplementations or BaselineStatus to a feature).
  • Verified that all transitions (Unset <-> Set) for names and baseline statuses trigger correct changes.