feat(export): classify metadata-described columns for volatile defaults

[pyramation]

Summary

@pgpmjs/export could only detect volatile-default columns for physically deployed tables: getDataExportColumns reads pg_attrdef + pg_proc.provolatile off a live relation. Callers with metadata-only planes (tables that exist as schema metadata but aren't physically deployed at export time) had no shared way to run the same classification, so they hand-rolled regexes over default-expression text.

This adds a first-class metadata-table entry point that runs the exact same provolatile-based classification:

interface MetadataExportColumn { name: string; type: string; columnDefault: string | null }

getMetadataExportColumns(db, columns): Promise<DataExportColumn[]>

Implementation builds an ephemeral CREATE TEMP TABLE pgpm_metadata_export_columns (c0 <type> [DEFAULT <expr>], ...) from the supplied descriptions, then calls getDataExportColumns on it — one shared code path. Results preserve input order/names, so callers reuse isVolatileTimestampColumn unchanged:

const volatile = (await getMetadataExportColumns(db, fields))
  .filter(isVolatileTimestampColumn);

Transactional & deterministic — zero residue. The whole classification runs inside a transaction that is always rolled back (BEGIN … ROLLBACK, or SAVEPOINT … ROLLBACK TO when the caller is already inside a transaction, e.g. pgsql-test). DDL is transactional in Postgres, so even a mid-flight failure erases the temp table — nothing is ever committed, the table name is a fixed deterministic identifier (no random suffix), and a failure cannot abort the caller's transaction. db must be a single session (client, not pool), as with any temp-table work.

Synthetic c0..cN column identifiers keep the DDL immune to reserved words / duplicate names. Each column's type must resolve in the connected DB and each non-null default must be a valid default expression for that type.

Notes

• Consumed by constructive-db's generate_constructive.ts (scoped-plane meta-export), which is replacing its regex fallback with this API — see constructive-io/constructive-db#2490. That consumer needs this published (@pgpmjs/export next minor, ^1.4.0) before it can adopt it.
• Tests: getMetadataExportColumns coverage in pgpm/export/__tests__/export-data.test.ts asserting parity with getDataExportColumns over an equivalent physical table, isVolatileTimestampColumn selection, input-order preservation, rollback cleanup, and the failure path (no leftover temp table, session still usable). Full package suite green.

Link to Devin session: https://app.devin.ai/sessions/b7e597395f8d4394a20a72f2473b6934
Requested by: @pyramation

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