Shared canonical export-name converter (camelCase to snake_case with override table)

[razbroc]

What to build

Add a shared canonical export-name converter to raster-shared that maps camelCase property keys to the snake_case column names used when writing GeoPackage export tables. It wraps change-case's snakeCase for the general case and consults a small override table for naming exceptions.

The override table's only entry today is resolutionDegree → resolution_deg (the per-part resolution field). Note the aggregate fields maxResolutionDeg/minResolutionDeg already abbreviate, so they need no override. The override table is the single, documented place where export-naming exceptions are declared.

Name the function to reflect that it is the canonical export-column-name mapping (e.g. an export-column-name converter), not a generic snake_case utility, so future consumers understand it carries overrides. This converter is intended to replace Overseer's private convertObjectKeysToSnakeCase and to be consumed by polygon-parts-worker.

No snake_case TypeScript type is produced — snake_case is derived at runtime only; the camelCase Zod schemas in raster-shared remain the source of truth for field names.

Release a new raster-shared version so downstream repos can consume it.

Acceptance criteria

• A converter exported from raster-shared maps an object's camelCase keys to snake_case keys, using change-case for the general case.
• Acronym/number tokens convert correctly: horizontalAccuracyCE90 -> horizontal_accuracy_ce90, imagingTimeBeginUTC -> imaging_time_begin_utc.
• An override table is consulted before the general rule; resolutionDegree maps to resolution_deg.
• Adding or removing an override is a single-location change.
• Unit tests cover the general conversion, the acronym/number cases, and every override entry.
• A new raster-shared version is released and publishable for downstream consumption.

Blocked by

• None - can start immediately.