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

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## 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.

Start by locating the raster-shared export surface, its existing tests, and the package release configuration; also inspect Overseer's private convertObjectKeysToSnakeCase and the polygon-parts-worker consumer. Add the shared converter and one documented override table, cover general, acronym/number, and override cases, then verify a new raster-shared version can be released for downstream use.

Written by the indexing model from the issue text.