rules: document tests outside rules/is, prefer plain rules

[samlown]

Docs-only change to rules/README.md, prompted by review of gobl.dk.oioubl#8, where a large number of is.Func helpers were written for checks the framework and domain packages already cover.

Tests exist outside rules/is

The "Available tests" section only documented rules/is, so there was nothing pointing addon authors at the tests each domain package already exports. Added a preference order (generic → domain → custom function) and a new Domain tests outside rules/is section inventorying every exported rules.Test in the repo:

• num — Positive, ZeroOrPositive, NotZero, Min/Max/Equals, .Exclusive()
• cal — DateNotZero/After/Before plus the DateTime and Timestamp equivalents
• cbc — InCodes, InCodeDefs, InKeyDefs, HasValidKeyIn, CodeMapHas
• tax — the six Extensions* rules, ExtensionHasValidCode, SetHasCategory/SetHasOneOf, RegimeIn, IdentityIn, HasAddon/AddonIn, TagsIn
• org, bill, currency, head, uuid — identity and attribute tests, the *TypeIn When guards, RequireLineTaxCategory, CanConvertTo, StampsHas, UUID version tests

Closes with a note asking authors to push generally-useful tests down into the type's own package rather than keeping them private to an addon.

Prefer plain rules over helper functions

New section leading "Common patterns", with three before/after pairs drawn from the patterns in that PR:

• is.Func that type-asserts to reach a nested value → nested rules.Field + AssertIfPresent(is.In(...))
• hand-rolled amount and tax-set predicates → num.NotZero, tax.SetHasCategory
• one helper spanning two fields → one assertion per field, so faults land on totals.payable and totals.due separately instead of collapsing onto totals

The point worth the most: a helper's val.(*bill.Tax) guard returns true on a type mismatch, so a field rename silently disables the rule with no test failure, whereas Field validates its name against the struct at initialisation and panics. The section ends by saying custom functions remain the right answer for genuine cross-field logic and domain lookups.

All three "Prefer" examples were compile-run against real bill.Invoice data before being documented — the five rules fire with exactly the codes and paths shown, and an empty rounding is correctly skipped.

Fault format correction

Found while verifying the above: the Testing section documented faults.HasPath("field") and an error format of [CODE] field: description. HasPath matches JSON Paths rooted at $ exactly, and Fault.Error renders [CODE] ($.field) description. Corrected both, and documented that faults sharing a code and message merge into one entry listing every path.

No CHANGELOG entry — docs only.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.87%. Comparing base (43ca367) to head (d922f50).

Additional details and impacted files

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##             main     #911   +/-   ##
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  Coverage   94.87%   94.87%           
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  Files         320      320           
  Lines       17431    17431           
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  Hits        16537    16537           
  Misses        553      553           
  Partials      341      341           

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