Refactor: all plugins use functional option pattern

[Claude]

Extends the functional option pattern introduced for plugin/singbox (commit 182fdc0) to every converter under plugin/, so converters can be built programmatically with validated options instead of raw struct literals.

Constructor shape

Each converter now exposes:

• New*(action, opts ...lib.InputOption) / (..., opts ...lib.OutputOption) — applies options, validates, returns the converter
• New*FromBytes(action, data []byte) — unmarshals JSON config and delegates to New*
• With* functions for each individual field

The concrete structs are unexported (snake_case: mrs_in, text_out, geolite2_asn_csv_in, cutter, …), so construction goes through the constructors.

input := plaintext.NewTextIn(
    plaintext.TypeTextIn,
    plaintext.DescTextIn,
    lib.ActionAdd,
    plaintext.WithNameAndURI(name, uri),
    plaintext.WithInputDir(dir),
)

Validation

Required/optional derived from configuration.md; invalid combinations abort via log.Fatalf in New*:

• name and uri are a single WithNameAndURI(name, uri) option — specifying one without the other is rejected
• inputDir required when neither name nor uri is given (clashRuleSet, clashRuleSetClassical, json, mihomoMRS, singboxSRS, surgeRuleSet, text)
• v2rayGeoIPDat input requires uri; json requires jsonPath; stdin requires name; cutter requires remove action plus non-empty wantedList; lookup output requires search

Option naming

Packages with a single in/out converter (mihomo, v2ray, singbox) use the generic names. Packages with several converters prefix per converter to avoid collisions:

• maxmind: WithMMDBIn*, WithASNCSV*, WithCountryCSV* for inputs; generic WithOutput* + WithSourceMMDBURI for the single output
• special: WithCutter*, WithPrivateOnlyIPType, WithStdin*, WithStdout*, WithSearch/WithSearchList
• plaintext: all input types share text_in and all output types share text_out, so generic names work with iType/iDesc passed to the constructor

Defaults

Type-dependent defaults (plaintext output dir/extension, maxmind default MMDB URI and CSV paths, maxmind output dir/name) are seeded in the constructor before options are applied; the matching With* is a no-op on empty input, preserving current behavior for configs that omit them.

Callers

Root lookup.go and merge.go now build converters via the new constructors rather than struct literals.

[Loyalsoldier]

@copilot Review this PR, try to refine it and find potential bugs.

[Loyalsoldier]

@codex[agent] Review this PR, try to refine it and find potential bugs.

[Copilot]

@copilot Review this PR, try to refine it and find potential bugs.

Reviewed and fixed the lookup --dir regression plus MRS/SRS source/default validation in 7563936; refinements are in a6d6e93 and 9c73aee. Full tests/build pass, and final review/CodeQL are clean.

[Codex]

@codex[agent] Review this PR, try to refine it and find potential bugs.

Reviewed the PR at 9c73aee and rechecked the functional-option constructors, lookup source selection, and validation paths. I did not find additional regressions to patch; go test ./... and go build ./ both pass locally.