[tests] Slim down testIDT.cpp — spot-checks for fixtures, fingerprints for transforms

## Summary

**`tests/testIDT.cpp`** embeds **large numeric table literals** next to tests that already load **`rawtoaces_data`** JSON under `DATA_PATH`. That duplicates on-disk data, bloats the file (thousands of lines), and is hard to review. This work replaces those patterns with **lighter assertions**: spot-checks for loaded fixtures, and **fingerprints** for transformation outputs.

## Background (for newcomers)

These tests exercise IDT-related loading and math. The intended reader is someone picking up a **day-sized** task (for example during [ASWF Dev Days](https://www.aswf.io/dev-days/)) who may not have touched this suite before. The goal is to preserve regression coverage while making **`testIDT.cpp`** easier to maintain—without adopting snapshot-testing tooling.

## Where to look

- **`tests/testIDT.cpp`** — large `double` / table literals interleaved with `OIIO_CHECK_*` assertions.
- Fixture root: **`DATA_PATH`** → `../_deps/rawtoaces_data-src/data/` (camera, CMF, and related JSON).
- Search for big brace-initialized arrays and loops comparing every row to a literal.

## Suggested direction (not a prescription)

**Fixture data (already on disk)** — When the test only checks that a JSON asset was read and parsed correctly:

- Assert **structure** (dimensions, channel keys, sizes).
- Compare **a few rows** only — e.g. **first** and **last** couple of rows of each relevant channel — instead of embedding the full table in C++.

**Computed / transformation output** — When the assertion is about pipeline output, not raw file bytes:

- Prefer a **stable fingerprint** (checksum or hash over a clearly defined serialization of the result).
- Store an **expected fingerprint** constant in the test and assert **computed fingerprint == expected**.

**Rough success criteria:** meaningful shrink of **`testIDT.cpp`** where literals were redundant; regressions still caught (bad dimensions, parse errors, broken transforms); fingerprints **deterministic** in CI (or platform quirks documented).

Read tests/testIDT.cpp and locate the large brace-initialized arrays and row-by-row comparisons. Check the corresponding JSON files under DATA_PATH/../_deps/rawtoaces_data-src/data/ to distinguish fixture checks from transformation outputs. Done means the redundant literals are substantially reduced while dimensions, parsing, spot-checks, and deterministic transformation fingerprints still protect the tests.

Written by the indexing model from the issue text.