pgxoption: nested module for native pgx v5 (pgtype) integration

[moznion]

What

Adds github.com/moznion/go-optional/pgxoption as a nested Go module (own go.mod, so the core package stays dependency-free; release by tagging pgxoption/vX.Y.Z).

pgxoption.Register(m *pgtype.Map) installs TryWrapScanPlanFunc/TryWrapEncodePlanFunc wrappers that plug Option[T] directly into pgx v5's plan machinery:

• Scan: wraps the destination as **T, so pgx's own NULL→nil-pointer handling applies; nil maps to None, a value maps to Some (a previous Some is overwritten by a NULL scan).
• Encode: None → SQL NULL; Some(v) delegates to T's registered codec.

Why

Without registration, pgx handles Option[T] via the generic sql.Scanner/driver.Valuer fallback, which detours through driver.Value — extra conversions, text-ish and lossy for types like numeric, and Option[T]'s underlying []T shape sits uncomfortably close to pgx's slice→array reflection. Native plans give full type fidelity in both text and binary formats for every element type the connection's pgtype.Map supports.

Detection is by defined type (PkgPath + Option[ name prefix), so plain slices keep flowing through pgx's array machinery untouched (pinned by a test).

Notes

• go 1.23.0 / pgx v5.7.6 — the oldest pgx line that doesn't force a newer Go directive on consumers (MVS lets users float pgx upward freely). CI gets a separate pgxoption job (1.23.x/1.24.x), since the root matrix still includes 1.22.
• Tests are offline against pgtype.NewMap(): text/binary scans, NULL→None, None→NULL encoding, a timestamptz binary round-trip, and the plain-slice non-interference pin.
• Root README gains a short pointer section.

Context: sqletch is adopting Option[T] across its generated code (moznion/go-sqletch#21). Its generated scan path deliberately does not require this module — pgxoption is the opt-in native-fidelity path for direct pgx users.

🤖 Generated with Claude Code

https://claude.ai/code/session_01P7UbMMpmzxDECU9gqxAAZ3

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (f8828b8) to head (55a3a47).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #54   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          211       168   -43     
=========================================
- Hits           211       168   -43     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.