Fix race condition in barrier seq range tracking

[rcarver]

Problem

There's a race condition between beginBarrier and the user's database write. The current implementation uses seq ranges to identify which undolog entries belong to a barrier:

1. beginBarrier reads max(seq) as startSeq
2. ⚠️ Another write could happen here → undolog entries created
3. User's write happens → undolog entries created
4. endBarrier reads max(seq) as endSeq, captures range [startSeq+1, endSeq]

If concurrent writes occur between steps 1 and 3, those unrelated undolog entries are incorrectly included in the barrier's range.

Proposed Solution

Tag undolog entries with a barrier ID rather than relying on seq ranges.

Schema change:

-- Add barrierId column to undolog
ALTER TABLE undolog ADD COLUMN barrierId TEXT;

-- Add currentBarrierId to undoState  
ALTER TABLE undoState ADD COLUMN currentBarrierId TEXT;

Trigger change:

INSERT INTO undolog(barrierId, tableName, sql) 
VALUES((SELECT currentBarrierId FROM undoState WHERE id = 1), ...);

API change:

• beginBarrier sets currentBarrierId in undoState
• endBarrier queries WHERE barrierId = ? instead of seq range
• Entries with barrierId = NULL are ignored (written outside a barrier)

Benefits

• Eliminates race condition entirely
• Works correctly with concurrent database access
• No change to public API