feat: configurable operators/directions, dotted fields, and docs restructure

[christiangda]

Summary

Three related additions, all backward compatible, plus a documentation restructure.

1. Configurable filter operators (opt-in)

Callers can restrict which operators a FilterParser accepts; the default (no option) still allows everything.

p := qfv.NewFilterParser(fields,
    qfv.WithAllowedOperatorGroups(qfv.GroupComparison, qfv.GroupLogical),
    qfv.WithAllowedOperators(qfv.OpIn),
)
p.Parse("age >= 18 AND status IN ('active')") // ok
p.Parse("name LIKE '%x%'")                     // error: operator "LIKE" is not allowed

• New Operator enum (per-verb) + OperatorGroup presets (GroupComparison, GroupPattern, GroupLogical, …), both additive.
• Helpers AllOperators() and OperatorGroup.Operators().
• Disallowed verbs are reported through the existing single-pass error aggregation.
• Negation semantics: negated predicates (NOT IN, NOT LIKE, IS NOT DISTINCT FROM, …) are governed by their base operator; OpNot governs only the standalone leading NOT (…).

2. Configurable sort directions (opt-in)

p := qfv.NewSortParser(fields, qfv.WithAllowedDirections(qfv.SortAsc))
p.Parse("name DESC") // error: sort direction "DESC" is not allowed

3. Nested dot-notation field names

FilterParser now accepts dotted identifiers via scanner.IsIdentRune:

p := qfv.NewFilterParser([]string{"user.profile.age", "user.name"})
p.Parse("user.profile.age >= 18 AND user.name = 'John'")

A dot is valid only inside an identifier, so numeric literals (3.14) are unaffected. FieldsParser/SortParser already matched dotted names (plain string comparison); this brings FilterParser in line. Unknown dotted fields are rejected by the allow-list as usual.

Constructors stay source-compatible — NewFilterParser(fields) / NewSortParser(fields) gained variadic options, so existing calls compile unchanged.

Documentation restructure

• Detailed guides moved into docs/: getting-started, filtering, configuration, error-handling, migration.
• Top-level README.md slimmed to essentials — install/update, a quick-start, a short feature list, and links into docs/.
• New godoc Example* functions for the new options and dotted fields.

Tests

• Operator gating (groups + individual, additive, negation), group-coverage invariant, sort-direction gating, dotted-identifier parsing, numeric-literal regression.
• go vet, gofmt, go test -race ./... all pass; coverage 96.6%.

All of this targets the same unreleased v0.1.0 line; the migration guide (docs/migration.md) documents it as additive items 7–8.