docs: correct confirmed factual errors in integration guides

[unixwhisperer]

Fixes verifiable documentation errors found during an end-to-end external integration, each checked against contract source, live mainnet reads, or the test suite:

• max-single-loan: corrected 5,000 STX -> 500,000 STX across INTEGRATION_GUIDE, API_REFERENCE, and TESTING_GUIDE_STX (live get-max-single-loan = u500000000000; the source literal in flashstack-stx-core is u500000000000)
• README: STX receiver impl-trait pointed at the wrong contract; the trait is at SP3TGRVG7DKGFVRTTVGGS60S59R916FWB4DAB9STZ.stx-flash-receiver-trait, so the snippet as written would not deploy
• TESTING_GUIDE_STX: get-stats field names corrected to the actual STX core fields (total-loans / total-fees), replacing total-flash-mints / total-fees-collected
• TESTING_GUIDE_STX: clarify that flashstack-core (SP3TGRVG7) is the legacy sBTC path while flashstack-sbtc-core (SP20XD46) is live and active
• TESTING_GUIDE_STX: STX is-approved-receiver returns a bare bool, not (ok bool)
• INTEGRATION_GUIDE: Clarity has no max builtin; use the raw-fee / if form
• INTEGRATION_GUIDE: the is-approved-receiver read-only call requires a serialized contract principal (0x06...), not the ascii-hex of the string
• README: standardize the test count to 82 (matches the badge and a full vitest run: 82 passed)

Description

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update
• 🧪 Test addition or improvement
• 🔧 Refactoring (no functional changes)
• 🎨 Style/formatting changes
• ⚡ Performance improvement

Related Issues

Fixes #
Related to #

Changes Made

Testing Performed

Manual Testing

• Tested locally with clarinet check
• Tested in clarinet console
• Tested specific edge cases

Automated Testing

• All existing tests pass
• Added new tests for changes
• Tests cover edge cases

Test Commands Run

# List the commands you ran to test
clarinet check
clarinet console3

Test Results

# Paste test output here

Checklist

Code Quality

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• Any dependent changes have been merged and published

Documentation

• I have updated the documentation accordingly
• I have updated the README.md (if needed)
• I have added/updated code comments
• I have updated the CHANGELOG.md

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested edge cases
• I have tested error conditions

Security & Safety

• My changes don't introduce security vulnerabilities
• I haven't committed any secrets or sensitive data
• I've considered the impact on existing users
• I've followed Clarity best practices

Screenshots (if applicable)

Additional Context

Deployment Considerations

• Requires contract deployment
• Requires data migration
• Requires configuration changes
• Backward compatible

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By submitting this PR, I confirm that:

• My contribution is made under the MIT License
• I have read and followed the CONTRIBUTING.md guidelines
• I am the original author or have permission to contribute this code

[vercel]

@unixwhisperer is attempting to deploy a commit to the matt glory's projects Team on Vercel.

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[mattglory]

Approved. All fixes verified against contract source and live reads. Note: the max-single-loan fix in API_REFERENCE.md, INTEGRATION_GUIDE.md, and TESTING_GUIDE_STX.md overlaps with a commit already on main — you may see a merge conflict there. Everything else is clean and correct. The whitelist curl encoding fix and the is-approved-receiver bare-bool clarification are particularly valuable.