Point commcare-core submodule at master branch

[dannyroberts]

https://dimagi.atlassian.net/browse/SAAS-19426

Product Description

No user-facing changes. Infrastructure cleanup only.

Technical Summary

Part of the formplayer branch elimination effort. The commcare-core formplayer branch has been deleted — both commcare-android and formplayer now use master, with cross-repo CI (commcare-core PR #1525) ensuring compatibility.

This PR:

• Updates .gitmodules to track master instead of the deleted formplayer branch
• Updates the submodule pointer to current master HEAD
• Updates README to reflect the single-branch workflow and cross-repo CI

Safety Assurance

Safety story

The commcare-core master and formplayer branches were already reconciled before the formplayer branch was deleted. This change points the submodule at the same code on a different branch name.

Automated test coverage

CI will verify that formplayer builds and tests pass with the submodule pointing at master.

QA Plan

• CI passes with submodule pointing at master

Special deploy instructions

• This PR can be deployed after merge with no further considerations.

Rollback instructions

• This PR can be reverted after deploy with no further considerations.

Review

• The set of people pinged as reviewers is appropriate for the level of risk of the change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.09%. Comparing base (37c58be) to head (dc45eee).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1767      +/-   ##
============================================
- Coverage     70.12%   70.09%   -0.03%     
+ Complexity     2030     2029       -1     
============================================
  Files           257      257              
  Lines          8009     8009              
  Branches        763      763              
============================================
- Hits           5616     5614       -2     
- Misses         2110     2112       +2     
  Partials        283      283              

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[dannyroberts]

Looking at the diff on the libs/commcare change, it looks like the only additional PRs that were merged into master but weren't yet on the formplayer branch at the time that we deleted it were dimagi/commcare-core#1521 and dimagi/commcare-core#1516.

(There are a few other PRs, but they are all PRs that only touch GHA workflows under .github/ and docs/ and so categorically could not break formplayer by incorporating them.)

[dannyroberts]

Tests have of course been run against this, so that adds some level of assurance, but just wanted to raise in case anyone wants to advocate for more deeper testing/QA.