Accelerate SQLite bulk inserts

[schapman1974]

Summary

This PR speeds up SQLite-backed bulk inserts without changing the MongoDB-compatible behavior exposed by TinyMongo.

• Adds exact-built-in BSON encoding and identity-key fast paths while preserving subclass handling for types such as Code and Binary.
• Prepares each SQLite insert once, reusing its serialized payload and typed physical _id across candidate checks, retries, and commits.
• Adds an optimistic initial-load path for empty, canonical SQLite tables.
• Avoids repeatedly probing collections already known to be nonempty.
• Expands coverage around ordered/unordered failures, retries, timestamps, typed IDs, custom mappings, schema changes, triggers, and unique indexes.

Why

Profiling showed that most bulk-insert time was spent above SQLite: repeated BSON checks, JSON serialization, identity-key construction, and candidate planning. The new prepared-row path removes that duplicate work. For a brand-new compatible table, the optimistic path also skips the general conflict planner after proving under a write transaction that the table is empty and safe.

Contract and safety boundaries

The optimistic path is deliberately narrow:

• It accepts only exact built-in document shapes that have already passed BSON validation.
• It is disabled for top-level zero timestamps that require server-time stamping.
• It begins an immediate transaction and rechecks the index catalog, schema, and emptiness inside that transaction.
• It is disabled for unique catalog specs, triggers, external unique indexes, legacy/custom table definitions, and noncanonical primary-key behavior.
• Any SQLite integrity failure, including commit-time failure, rolls back and falls back to the existing Mongo-compatible planner.
• Nonempty collections retain the prepared general path, including ordered and unordered error semantics.

Local benchmark

Median local throughput for 10,000-document batches:

Engine/path	Inserts/sec
TinyMongo SQLite on master	13,123
TinyMongo SQLite, nonempty prepared path	63,486
TinyMongo SQLite, initial empty-table path	70,250
MongoDB (w=1, j=true)	108,890
Direct SQLite	258,821

The initial-load path is about 5.35x faster than the previous TinyMongo SQLite baseline and about 19% faster than the prepared-only stage. MongoDB results were noisier across runs; its comparison explicitly used acknowledged, journaled writes. Direct SQLite is an implementation ceiling rather than a contract-equivalent comparison.

Validation

• Full suite: 3744 passed, 1 skipped, 531 deselected
• Coverage: 100% across 8,314 statements and 3,524 branches
• ruff check .
• black --check .
• git diff --check