fix: bound semantic keyset scans

[acidkill]

Summary

• Read neuron and synapse keyset pages through bounded, ordered record-ID windows.
• Preserve cursor exclusivity, brain/time filters, and full-page behavior across window boundaries.
• Cover numeric-only and underscore-prefixed IDs with live SurrealDB 3.2.4 plan/parity regressions.

Why

SurrealDB 3.2 was scanning the full remaining suffix of neuron/synapse tables for each keyset page. A single open-ended record range changed the operator to RecordIdScan but still visited most of the suffix. These bounded adjacent leading-character ranges cap each scan interval; the original exclusive ID predicate remains authoritative, and pages continue into the next interval when the prior one has too few matching rows.

Test plan

• pytest tests/ -m "not stress" -n auto passes locally (7,748 passed, 179 skipped, 1 xfailed).
• ruff check src/ tests/ clean.
• ruff format --check src/ tests/ clean.
• mypy src/ --ignore-missing-imports clean.
• Disposable local SurrealDB 3.2.4: returned-ID parity, multi-brain/reference-time filters, unusual IDs, page fill across windows, and EXPLAIN FULL RecordIdScan.

Verified by

@acidkill.