Remove duplicate METATRAITS_DIR definition in constants.py

[turbomam]

Context

PR #502 resulted in METATRAITS_DIR being defined twice in constants.py:

• Line 33: METATRAITS_DIR = TRANSFORM_UTILS_DIR / METATRAITS
• Line 576: METATRAITS_DIR = TRANSFORM_UTILS_DIR / METATRAITS

METATRAITS_MAPPINGS_DIR is derived from the first definition (line 34). Having two definitions means future edits could update one but miss the other.

Action

• Remove the duplicate definition at line 576
• Keep METATRAITS_RAW_DIR (line 577) near the other raw dir definitions but derive it from the single METATRAITS_DIR

Ref: #502

[turbomam]

Status check (2026-03-27): constants.py moved to kg_microbe/transform_utils/constants.py. The duplicate METATRAITS_DIR is still there:

Line 33: METATRAITS_DIR = TRANSFORM_UTILS_DIR / METATRAITS
Line 34: METATRAITS_MAPPINGS_DIR = METATRAITS_DIR / "mappings"
...
Line 576: METATRAITS_DIR = TRANSFORM_UTILS_DIR / METATRAITS

Fix is still: delete line 576, keep METATRAITS_RAW_DIR (line 577) near the other raw dir definitions.