Add ISO week validation with inclusive bounds

[strider2038]

Adds ISO week validation for values such as 2020-W53, rejecting nonexistent weeks such as 2021-W53. it.IsWeek().WithMin("2020-W01").WithMax("2021-W01") supports inclusive ranges across ISO years; validate.Week and is.Week expose standalone validation with matching options.

• Strict YYYY-Www format, calendar-aware week 53 validation, and separate format, week-number, minimum, and maximum violations.
• Custom errors/messages, conditional validation, groups, generic This/Each, and English/Russian translations.
• No new dependencies. Years 0000–9999 use Go's proleptic Gregorian calendar literally. README documents differences from Symfony: empty strings are skipped, and early years are not remapped by PHP mktime rules.
• Malformed, nonexistent, or reversed bounds produce configuration errors. Includes examples and an Unreleased changelog entry.

Validation: bash scripts/test-all.sh (formatting, vet, golangci-lint v2.13.2, unit tests, race tests, module tidy and verification). Tests cover strict parsing, cross-year bounds, all four violations, custom messages, translations, and week 53 for every year 0000–9999 against the independent January 1/leap-year rule.

Closes #108.