[PM-37940] Update guidance on field keys and authoring map major version changes

[jprusik]

🎟️ Tracking

PM-37940

📔 Objective

Update guidance on field keys and authoring map major version changes

Additional context:
#18 (comment)
#11 (comment)

[github-actions]

Bitwarden Claude Code Review

Overall Assessment: APPROVE

This PR adds an "Authoring Maps → Major Version Bumps" section to the root README documenting the cross-Map workflow for shipping a new major schema alongside the previous one, consolidates the Forms Map field key reference into a single combined table with clarifying prose on role/input semantics, and adds two fail-fast build invariants in scripts/build.mjs (schema filename major must agree with properties.schemaVersion.const major, and $id must end with the schema filename). Verified the documented workflow matches the existing build/release behavior, the 43-entry consolidated field key table is in lockstep with the fieldKey enum in forms.v0.schema.json, all referenced anchors (#schema-versions, #prerelease-maps, #backwards-compatibility, #mapping-philosophies) exist in the README, and the TOC anchor fix raised by @bensbits91 was applied in commit f63383e. No new findings.

Code Review Details

No findings.

[jprusik]

Real world example which informed these changes and clarifications:

[bensbits91]

One minor issue with readme TOC after changing headings. Looks good otherwise

[bensbits91]

TOC needs to be updated to fix broken links after changing headings. In the TOC, around lines 40-42

Old:

      - [Name](#name)
      - [Contact](#contact)
      - [Address](#address)

New:

      - [Names](#names)
      - [Contacts](#contacts)
      - [Addresses](#addresses)

[jprusik]

ooo, nice catch - fixed in f63383e