fix: stage the qualify merge in a session-scoped temporary table

[pquadri]

Summary

Follow-up to #35 (review comment by @giamo): if the pod is killed while copying into <table>_temp_<id>, nobody cleans the table up and it is untagged.

_merge now runs the whole load in one Snowflake session and the staging table is TEMPORARY: only that session sees it and Snowflake discards it with the session. The manual drop-on-failure path is gone.

Changes

• Table.temporary adds TEMPORARY to the create-table and qualify statements (non-temporary SQL is unchanged).
• _merge opens one connection and publishes it through a ContextVar; the existing connect() sites in table.py reuse it, so copy_into, copy_custom, merge and merge_custom need no signature changes.

Review notes

• A killed process does not end its Snowflake session immediately: the temp table is invisible to everyone else and is dropped when that session expires (idle timeout, hours by default), not on the spot.
• The whole load now holds one connection for its duration (previously several short ones).
• Tests: 77 mocked tests pass. Also run against real Snowflake in a throwaway sandbox schema: first load, incremental load (0 duplicate reads, no temp table visible to a reader), forced COPY failure, a hard-killed process (nothing visible afterwards, live table unchanged), merge() path and schema evolution.

https://claude.ai/code/session_011PFLPuv77fp5M3JXdZvW1v

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Note

Medium Risk
Changes core load/merge connection lifecycle and staging table semantics; failures no longer explicitly drop temp tables but rely on Snowflake session expiry, and loads hold one connection for longer.

Overview
Qualified copy_into / merge staging no longer leaves orphaned <table>_temp_<id> tables if a worker dies mid-load. _merge now holds one Snowflake session for copy → qualify → merge, and the staging table is created as a session-scoped TEMPORARY table via a new Table.temporary flag (DDL and qualify emit TEMPORARY when set).

A ContextVar plus _connection() lets bulk_insert, _copy, and related paths reuse that session without API changes. The old manual DROP on failure path is removed; cleanup relies on session end. README documents the single-session temporary staging behavior. Tests assert temporary DDL, single connect(), and that failures close the session without merging.

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