refactor: Split LoanInvariant into LoanBrokerInvariant and LoanInvariant

[Tapanito]

Separate ValidLoanBroker and ValidLoan into their own header/source file pairs, following the existing pattern of other invariant files (e.g. AMMInvariant, VaultInvariant).

High Level Overview of Change

Context of Change

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[xrplf-ai-reviewer]

Two unguarded at() calls in the invariant checker (lines 101, 115) can throw on malformed SLEs — see inline comments.

Review by Claude Opus 4.6 · Prompt: V12

[codecov]

Codecov Report

❌ Patch coverage is 91.25000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.5%. Comparing base (509677a) to head (1776f1f).

Files with missing lines	Patch %	Lines
src/libxrpl/tx/invariants/LoanBrokerInvariant.cpp	91.2%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #6674   +/-   ##
=======================================
  Coverage     81.5%   81.5%           
=======================================
  Files          998     999    +1     
  Lines        74456   74456           
  Branches      7578    7555   -23     
=======================================
+ Hits         60648   60652    +4     
+ Misses       13808   13804    -4     

Files with missing lines	Coverage Δ
include/xrpl/tx/invariants/InvariantCheck.h	100.0% <ø> (ø)
src/libxrpl/tx/invariants/LoanInvariant.cpp	63.6% <ø> (-19.5%)	⬇️
src/libxrpl/tx/invariants/LoanBrokerInvariant.cpp	91.2% <91.2%> (ø)

... and 4 files with indirect coverage changes

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[bthomee]

@Tapanito Although a structural change only, as you told the AI bot, it would be a good idea to add:

1. An LCOV exclusion, and
2. An UNREACHABLE statement

here and below, if these places are not reached via unit tests / cannot be reached ever.

It's fine to do this in a follow-up PR, and probably should be part of a larger, comprehensive effort to do this properly across all invariants.

[Tapanito]

Thanks @bthomee, I will do this as part of Lending Protocol invariants. They are a little bit scant at the moment.

[xrplf-ai-reviewer]

One architectural concern flagged inline — keylet::unchecked inside an invariant checker lacks a rationale comment.

Review by Claude Opus 4.6 · Prompt: V12

[xrplf-ai-reviewer]

keylet::unchecked weakens invariant safety — directory entries can hold heterogeneous types, but add a comment explaining why unchecked is required here:

        // keylet::unchecked is intentional: the directory may hold either
        // ltRIPPLE_STATE or ltMPTOKEN entries; no single typed keylet covers
        // both.  The type is validated immediately below.
        auto const sle = view.read(keylet::unchecked(index));