Tidy README and document Postgres native-enum interop

[dmytro-khmara]

Summary

Same readability pass applied across the StrEnum family (StrEnum, StrEnum.EntityFrameworkCore, StrEnum.AspNetCore, StrEnum.System.Text.Json), plus a new section flagging the native Postgres enum interop story.

Tidy

• Allows to use → Lets you use; trailing-period fix on the support line.
• You can install ... using → Install ... via.
• New ### Defining a string enum and an entity heading so the first usage block isn't headless.
• Tightens the assembly-loading paragraph and the IN-clause workaround prose; aligns code-sample formatting.
• Bumps the copyright year (2022 → 2026) and corrects the author name (Dmitry Khmara → Dmytro Khmara) to match the rest of the family.

New: Postgres native enum columns

Documents that StrEnum.Dapper's text-flattening handler hits 42804: column ... is of type my_enum but expression is of type text against native PG enum columns, since Postgres has no implicit text → enum cast. Two practical workarounds are shown:

• Add ::your_enum to the parameter site in SQL.
• Pass a hand-rolled NpgsqlParameter with DataTypeName set, via DynamicParameters.

Also points raw-Npgsql-only consumers at StrEnum.Npgsql, and notes that there isn't a first-class StrEnum.Npgsql.Dapper integration today.

No code changes.

Test plan

• Render the README and verify formatting / new section
• Confirm the PG-enum workaround code samples are syntactically correct