feat(querybuilder): add typed composition and dialect-safe functions

[relsunkaev]

This lands the querybuilder capability work, including production wiring, documentation, and live PostgreSQL/MySQL/SQLite coverage.

Why

The builder was missing a composable path for dynamic queries, typed SQL fragments, reusable execution, and richer result contracts.

The original function surface also treated several dialect-dependent behaviors as portable. That made result types and runtime semantics look more consistent than the databases actually are.

New Features

Composition and execution:

• typed SQL fragments preserve dependencies, result types, and parameter ordering
• conditional selections and predicates compose through the existing pipeable query workflow
• prepared handles reuse rendered plans
• cardinality helpers stay pipeable instead of expanding the executor method surface
• EXPLAIN and mutation metadata retain dialect-owned behavior

Functions and types:

• PostgreSQL, MySQL, and SQLite own native aggregate, modulo, round, case-conversion, clock, and explicit-frame semantics
• root functions keep only the cross-dialect contract
• count and ranking values decode as BigIntString
• numeric operations validate database types rather than only TypeScript runtimes
• portable type witnesses move from Query.type to Type

Breaking Changes

• use execute(...).pipe(Executor.atMostOne/exactlyOne/nonEmpty) instead of executor cardinality methods
• Query.keyset is removed pending a reviewed pagination contract
• use Type.<kind>() instead of Query.type.<kind>()
• root Function no longer exports native sum, avg, case-conversion, clock, modulo, or round helpers
• explicit aggregate and value window frames use dialect Function modules
• count and ranking results are branded bigint strings

Risks & Testing Needed

Risk: consumer compile failures from the intentional API removals
→ the changelog and README show the replacement imports and pipeable calls

Risk: numeric result mismatches across drivers
→ live tests assert exact, approximate, integer, zero-divisor, aggregate, and rounding behavior on all three engines

Reversed/removed features:

• the first compound keyset API was removed rather than shipped without an explicit cursor and ordering contract
• nonportable standard function exports were moved to dialect modules

Build status: pass locally

Test Plan

bun run test:types
bun test
bun run test:integration

Results:

• 550 unit and behavior tests passed
• 70 live integration tests passed
• package build and packed consumer smoke passed

Rollback

git revert <merge-commit>

This restores the previous public surface; consumers should not adopt the new APIs until the release is confirmed.