docs: explain how to use the Go 1.27 stdlib uuid package via overrides - #4599

Go 1.27 added a uuid package to the standard library (#4590). This documents the type overrides that let users adopt it today, ahead of sqlc switching its default UUID type:

overrides:
  - db_type: "uuid"
    go_type: "uuid.UUID"
  - db_type: "uuid"
    nullable: true
    go_type:
      import: "uuid"
      type: "UUID"
      pointer: true

Nullable columns map to *uuid.UUID since the standard library has no NullUUID type. No code changes were needed: the override machinery already parses the stdlib import path, and the existing special-casing in imports.go suppresses the hardcoded github.com/google/uuid import whenever an override renders as uuid.UUID.

Verified end-to-end against PostgreSQL 16 with the generated code compiled under go1.27.0, for both sql packages:

• pgx/v5 handles the type via its underlying-[16]byte wrap plans (TryFindUnderlyingTypeScanPlan / TryWrapFindUnderlyingTypeEncodePlan).
• database/sql gained first-class support in Go 1.27: driver.DefaultParameterConverter binds uuid.UUID parameters and convertAssign scans string/[]byte values into uuid.UUID and *uuid.UUID destinations, with nil as NULL.

Round-trips covered v4 and v7 UUIDs plus NULL and non-NULL nullable columns.

No endtoend testdata case is included because generated code importing the stdlib uuid package requires Go 1.27+, and the testdata module builds under the repo's Go 1.26 toolchain. uuid[] array columns were not verified, so the docs make no claims about them.

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https://claude.ai/code/session_01N9jm9kAWEiXHxg3LGsNP4p

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