feat(sdk): add Context.withSchema schema-scoped contexts

[damusix]

Summary

• ctx.withSchema<SDB, SProcs, SFuncs, STvfs>(name) derives a schema-scoped Context sharing the parent's connection, pool, and lifecycle — the query builder, proc/func/tvf, transactions, and impersonation all resolve against the schema, with fresh generics typing the schema's slice. Re-deriving replaces rather than stacks; dotted routine names keep the caller's qualification; raw sql fragments stay untouched by design.
• Proven live by a three-schema fixture per dialect (schemas on postgres/mssql, databases on mysql, ATTACHed databases on sqlite) with interleaved cross-context isolation assertions, plus compiled-SQL unit coverage.
• Ships a @noormdev/sdk minor changeset and documentation across the SDK README, API reference, and dev guide.

What this solves

Multi-schema users had to hand-qualify every query and routine call; one missed call site silently targeted the default schema.