fix(honeydew): honor a field's declared datatype in Ossie → Honeydew

[baruchoxman]

Summary

Fixes #411. _ossie_field_to_honeydew_datatype derived the Honeydew datatype only from the shape of dimension, so a Date field became number and a Boolean became string — the declared datatype had zero effect on the output. The heuristic dates back to the converter's first commit, when the spec had no datatype field; #113 added the enum and the converter was never updated.

What changed

• converter.py: added _OSSIE_TO_HONEYDEW_DATATYPE, mapping the core-spec datatypes enum onto Honeydew's vocabulary. _ossie_field_to_honeydew_datatype now consults, in order: the round-tripped HONEYDEW extension datatype → the field's declared datatype → the existing dimension-shape heuristic. Opaque, absent, and unrecognized values fall through to the heuristic, so models that declare no datatype convert exactly as before.
• README.md: replaced the now-false "Ossie fields have no explicit datatype" limitation with the actual mapping, and documented the two limitations that remain (see below).
• Tests: all nine spec datatypes, the fallback cases (Opaque, unrecognized, non-string), extension-vs-declared precedence, an end-to-end file-content case, and a _fields_to_honeydew case covering both source and calculated attributes.

Mapping notes

Two deliberate deviations from the mapping suggested in the issue, both per Honeydew's attribute schema, where number means integer and float means floating point:

• Float → float (not number) — number would claim it is integral.
• Decimal → float (not number) — Honeydew has no exact-decimal type, and Decimal is not integral either, so float is the closer approximation.

Per the spec, dimension.is_time is a role flag independent of type, so a year-grain field (datatype: Integer, is_time: true) now yields number rather than timestamp. That is the intended consequence of letting the declared type win.

Out of scope

• metric.datatype is still ignored — metrics are hardcoded to number. Same bug class, but Ossie->Honeydew converter ignores a field's explicit datatype #411 scopes to fields, and Honeydew metrics have no time type. Documented as a limitation in the README.
• test_ossie_roundtrip_tpcds_example fails on this branch, and identically on unmodified main — this converter still expects the semantic_model: wrapper that Define one semantic model per document without a wrapper #383 removed. That is Update remaining converters for the flat (one-model-per-document) Ossie schema #418, and fix(honeydew): read and write flat semantic model documents #420 fixes it for this converter. fix(honeydew): read and write flat semantic model documents #420 is independent of this PR and should merge first; once it lands I will rebase this branch on main and that failure disappears. Until then this PR's CI shows that one red test, inherited from main. The two branches merge cleanly in either order — verified by cherry-picking this commit onto fix(honeydew): read and write flat semantic model documents #420's branch: 145 passed.

Test plan

• cd converters/honeydew && uv sync && uv run pytest — 147 passed, 1 pre-existing failure (test_ossie_roundtrip_tpcds_example, red on main too, tracked by Update remaining converters for the flat (one-model-per-document) Ossie schema #418)

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