fix: distinguish collection and element matches when walking input

[wolveix]

Follow-up to #1076, which surfaced two silent issues in the input walker.

Resolver on a named collection type returned a 500. type Coords [2]float64 with a Resolve method: everyPB treated every collection as a container to walk through, so it stepped past the array and tried to run a resolver on each float64. The cause is that _findInType recorded the same path whether the match was on the collection's own type or on its element type. It now records a marker step when descending into elements, so both are exact and the kind-based guess is gone. This also fixes the case where a collection and its element both have resolvers, which previously ran the element's twice and the collection's never.

Defaults and resolvers reached through a map panicked with reflect: reflect.Value.Set using unaddressable value, because map values are copies. The value is now copied out, walked, then written back with SetMapIndex, so defaults and resolver mutations inside maps stick. map[string]Struct with a default: tag panics on main today, so that is fixed too.

Tests cover resolvers on named array, slice and nested collection types, collections whose elements also resolve, and defaults plus resolver mutations through maps. Each fails without these changes. Benchmarks are flat except map bodies, about 4% for the write-back.

Known limitation, not addressed here: an omitted optional [N]T field runs resolvers and defaults on N zero elements, since a Go array is always N values. That matches existing behaviour for non-pointer struct fields; []T and *[N]T work as expected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.20%. Comparing base (5961f0a) to head (dc51cbb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1082      +/-   ##
==========================================
- Coverage   93.22%   93.20%   -0.03%     
==========================================
  Files          23       23              
  Lines        4976     4988      +12     
==========================================
+ Hits         4639     4649      +10     
- Misses        271      272       +1     
- Partials       66       67       +1     

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