Add icebergUpdateSchema for schema evolution

[platypii]

Fixes #25

Exposes the existing add-schema / set-current-schema commit logic through a stable public entry point, so consumers can add nullable columns (or rename columns, promote types) to an existing table in place — no recreate-and-reload.

Changes

• icebergStageUpdateSchema({ metadata, schema }) (stage.js) — pure, metadata-only stage emitting add-schema (spec sentinel schema-id: -1) + set-current-schema, guarded by assert-table-uuid, assert-current-schema-id, and assert-last-assigned-field-id CAS requirements. Runs applyUpdates eagerly so evolution violations (field-id reuse, illegal type promotion, required column without defaults) fail at stage time rather than only at commit / server-side.
• icebergUpdateSchema({ catalog, ..., schema }) (write.js) — top-level commit through commitWithRetry; retry-safe since it writes no data files. Works for file and REST catalogs.
• StagedCommit supertype (types.d.ts) with optional snapshot for metadata-only commits; StagedUpdate extends it and keeps snapshot required for the snapshot-producing stages, so existing callers and tests are unaffected.
• Exported from index.js, documented in the README.

API

The caller passes the complete evolved schema with field ids assigned: existing columns keep their ids, new columns use ids above the table's last-column-id (matches HypAware's mergeFieldIdsFromTable). The new schema's id is assigned at commit time.

await icebergUpdateSchema({
  catalog, tableUrl,
  schema: {
    type: 'struct',
    'schema-id': 0, // ignored; assigned at commit
    fields: [
      ...schema.fields,
      { id: 3, name: 'score', required: false, type: 'double' },
    ],
  },
})

Acceptance criteria from #25

• ✅ Add a nullable column to an existing table and commit it, no recreate
• ✅ Subsequent appends use the evolved schema (staging reads current-schema-id)
• ✅ Pre-existing files read the new column as null (existing read-path null-fill)
• ✅ Field ids preserved for existing columns, assigned by caller for new ones
• ✅ Round-trip test: create → append → add column → append → read all back, old rows null in the new column

Testing

7 new tests in test/write/update-schema.test.js: the round-trip above, rename-by-field-id, rejection of incompatible type changes and defaultless required columns, and the staged requirements/updates shape. npx tsc, npm run lint, and all 598 tests pass.