feat(typespec): nested OBJECT/STRUCT schema fidelity (item 102)

[discreteds]

Summary

Implements item 102: recursive FieldSpec.object_fields for nested OBJECT/STRUCT schema fidelity.

• Adds recursive plain-dict and Frictionless x-mountainash.object_fields round-trips.
• Resolves nested struct dtypes for Polars, PyArrow, Narwhals, and Ibis schema strings; preserves Pandas' bare object representation.
• Extracts nested struct fields recursively from Polars and PyArrow, with backend_type=None for struct fields so resolver round-trips remain valid.
• Adds Polars-only conform stage-5e STRUCT materialization with nested inner casts and explicit non-Polars DtypeMappingError coverage.
• Registers and documents MA-CONF-04 for ibis-sqlite struct construction limitations; regenerates coverage artifacts.
• Marks the central design spec implemented.

Verification

• Full suite: 23106 passed, 81 skipped, 3470 xfailed, 49 warnings.
• Combined touched-test selection: 5852 passed, 57 skipped, 670 xfailed.
• Nested struct smoke scenario: ALL SMOKE CHECKS PASSED.
• Ruff: passed on modified files.
• Mypy: reports six pre-existing diagnostics in optional-library stubs and existing extraction annotations; no diagnostics in the new resolver/conform paths.

Notes

Conform STRUCT materialization intentionally remains Polars-only, matching the design and existing LIST/category conform precedent. Cross-backend tests assert DtypeMappingError off Polars; ibis-sqlite is marked with MA-CONF-04 because it cannot construct struct-typed tables.