feat: add RegisterIntEnum for integer-based enum flags

[leodido]

Description

RegisterIntEnum[E ~int | ~int8 | ~int16 | ~int32 | ~int64] registers an integer-based enum type for automatic flag handling via enumflag/v2. Mirrors RegisterEnum[E ~string] (#115) but targets signed integer enum types.

After registration, struct fields of type E work without flagcustom:"true" or manual Define/Decode/Complete hook methods.

Unsigned integer types are not supported (documented in the doc comment).

Usage

type Priority int

const (
    PriorityLow    Priority = 0
    PriorityMedium Priority = 1
    PriorityHigh   Priority = 2
)

func init() {
    structcli.RegisterIntEnum[Priority](map[Priority][]string{
        PriorityLow:    {"low"},
        PriorityMedium: {"medium", "med"},
        PriorityHigh:   {"high"},
    })
}

type Options struct {
    Priority Priority `flag:"priority" flagdescr:"task priority" default:"low"`
}

Features

• Case-insensitive matching and alias support (via enumflag/v2)
• Automatic {val1,val2,...} description enhancement (sorted by integer value)
• Automatic shell completion for all EnumValuer flags
• JSON Schema enum annotation
• Works with flagenv:"true", flagenv:"only", flagrequired:"true", default:"..."
• Panics on duplicate registration, empty values map, or alias collisions

Commits

1. feat(internal/hooks): DefineIntEnumHookFunc and StringToIntEnumHookFunc — define/decode hook factories for integer enums using enumflag/v2. Widens enumHelpText constraint to cover all integer widths.
2. feat: Public RegisterIntEnum + integration tests.
3. fix: Address review findings — fix broken doc comments (2.1), add alias collision panic to StringToIntEnumHookFunc (2.2), add env-var/env-only tests (2.3), validate empty values (2.7), document ordering/unsigned exclusion (3.1/3.3), fix enumHelpText int64 truncation on 32-bit (3.2).

Stacked on #115 — review that PR first.

How to test

go test -run "TestRegisterIntEnum|TestDefineIntEnum|TestStringToIntEnum" -v . ./internal/hooks/...
go test ./...