feat(typespec): canonical dtype schema declarations (333)

[discreteds]

Behavior

• Accept canonical dtype-only and compatible broad-type declarations; persist tagged enums and decimals.
• Honor native overrides with mismatch/unverified warnings. Native objects retain parameters in owned in-memory snapshots and are recursively rejected by descriptor serialization.
• Share effective targets across schema lowering, conform, inference, nested validation and ingress.
• Preserve explicit JSON/native-duration distinctions and existing decimal conversion policy.

Verification

• Full registered-backend quick suite: 32538 passed, 159 existing skips, 2490 xfailed, one schema-only inference failure. That in-repo regression was then repaired and verified by 353 assessment/inference tests; a subsequent selected-Narwhals native-carrier repair passed 69 source-shape/inference tests. The complete suite was not rerun after these final repairs.
• Final focused conform/converter/ingress owners: 532 passed, 21 retained xfails; source lint passes.
• All nine execution identities exercised for I8/U8/FP32 physical schemas and values: 23 passed, four nullable pandas cases remain xfailed under item252. Native-override nullable cases retain the same upstream failure.
• Actual typed Parquet write/readback: 15 physical-schema/value cases, plus JSON Files resource-to-DAG I8 readback, nested empty/null-only materialization and native timestamp/validation/ingress smokes.
• Native model/record ingress, wrong-owner rejection, decimal override precedence, categorical/timezone freeze, overflow and JSON/native-duration semantics exercised through actual public calls.
• Independent correctness and complexity reviews completed; all reported in-repo defects repaired. No static type checks run.

Approved limitations

On 2026-10-17 the owner explicitly approved retaining pre-existing item252 (NumPy-backed pandas float/NaN to integer through Narwhals) as nonblocking for item333, like the previously approved temporal-unit limitation in item334.

Both issues remain open and unrepaired. Original failing inputs and narrow strict xfails remain; no workaround, automatic ingress normalization, or claim of passing nullable acceptance is introduced. This clears the item252 draft gate only. Verification evidence above is unchanged, and the approval does not authorize a merge or close item333 before merge.

[discreteds]

Merged develop (1e4166f) into this branch and pushed conflict resolution d431d24. Preserved item333 native override evidence alongside item311 detailed/native-first schema resolution. Fixed evolve fallback inference losing detailed source metadata; SQLite schema assertions now distinguish physical affinities from narrow typed egress without removing output-width checks. Verification: 11,613 passed, 10 skipped, 182 xfailed across TypeSpec, conform, relations, core dtypes, validation-plan and pydata suites with full backend scope. Ruff passed. Runtime smokes exercised override precedence, nanosecond preservation, nested narrow dtype Parquet round-trip, and evolve values/schema/drift consistency. Approved nonblocking backlog items 252 and 334 remain unchanged.