Update README to reflect v1.3.0 changes

[lessthanjacob]

Summary

Updates the README to reflect changes shipped in v1.3.0 and fixes a stale CI badge.

Changes

• Fix CI badge branch. The Test badge was querying ?branch=master, but the default branch is main. Updated so the badge reflects current CI state.
• Document Symbol#to_proc shorthand for fields and identifiers (#546). Added a note in both the "Defining A Field Directly In The Blueprint" and "Defining An Identifier Directly In The Blueprint" sections showing the field :foo, &:bar / identifier :foo, &:bar form, with a caveat that options aren't available and the configured extractor is bypassed. Also points users toward name: for pure renames so the DSL remains consistent.
• Document Proc support for association :options (#579). Added an example to the Associations section showing options: ->(parent) { ... } for cases where options need to be derived from the parent object at render time.

No functional code changes — docs only.

Test plan

• Verify the CI badge renders correctly on the rendered README
• Verify the #renaming anchor link in the field section resolves
• Skim the rendered Markdown for formatting/collapsible-section correctness

Checklist:

• I have updated the necessary documentation
• I have signed off all my commits as required by DCO
• My build is green