fix: read table changefeeds by paging the database feed

[RobertSigmundsson]

Summary

• Semantic-link source fences no longer rely on SHOW CHANGES FOR TABLE ... LIMIT n returning n entries of that table.
• New _show_table_changes(table, since, limit) pages SHOW CHANGES FOR DATABASE and keeps the table's changes, so a short result really means the end of the feed. The barrier search, the quick source check and the paged source check all use it.
• Adds three live-SurrealDB regression tests and a CHANGELOG entry.

Why

On SurrealDB 3.2.3, LIMIT in SHOW CHANGES FOR TABLE is applied to the database-wide feed before the table's entries are kept: after 200 writes to table x, a write to y only shows up with LIMIT 250. The three readers in semantic_source_revision.py assume the limit counts the table's own entries. On a busy database that breaks all three: the barrier search takes a short page as the end of the feed, so semantic_link stops with "semantic source barrier was not found within the bounded changefeed scan (3 rows)"; the quick source check (LIMIT 10) misses neuron/synapse changes queued behind other tables' writes and accepts a stale token; the paged source check stops at an empty page. On a quiet database the marker sits in the first page, which is why this doesn't show in small tests.

Test plan

• pytest tests/ -m "not stress" -n auto passes locally.
• ruff check src/ tests/ clean.
• mypy src/ --ignore-missing-imports clean.
• tests/integration/test_surrealdb_semantic_source_revision.py (live SurrealDB 3.2.3, SURREALDB_URL): three new tests put 10-30 other-table writes ahead of the change each reader must see. On v3.12.0 they fail ("DID NOT RAISE SemanticSourceChangedError", "semantic source barrier was not present in its changefeed"); with the fix they pass, together with the existing changefeed tests (34 passed).
• A copy of a large, busy database: semantic_link fails on v3.12.0 with the message above; with the fix it completes (three pauses across the 600 s budget, each resume passing the source check).

The existing unit fake models per-table feeds, so it now overrides _show_table_changes; the database-feed paging itself is covered by the live tests.

Verified by

@RobertSigmundsson.