feat(vm): Comparable residuals + strict Time/Date parsers

[tannevaled]

Horodate: 2026-08-10 08:42 CEST

Adds the strict require 'time' / require 'date' class-method parsers, and locks in the Comparable residuals (already provided by the prelude) behind an explicit MRI-4.0.5 test.

(A) Comparable residuals

clamp (2-arg + Range incl. beginless/endless; exclusive-range and reversed-bound ArgumentError, non-Range TypeError), between?, and the <=>-derived operators < <= > >= == — including the exact ArgumentError "comparison of X with Y failed" when <=> is nil, and == staying lenient. These already lived in internal/vm/prelude.rb; this PR verifies them against ruby 4.0.5 (including that == propagates a raising <=> exactly as MRI does) rather than re-implementing.

(B) strict Time / Date / DateTime parsers

internal/vm/strict_parsers.go — each accepts only its wire format (a fixed set of Go reference layouts tried in order) and raises MRI's exact error otherwise:

method(s)	grammar	malformed error
Time.iso8601 / .xmlschema	YYYY-MM-DDTHH:MM:SS(.fff)?(Z|±HH:MM)?	ArgumentError "invalid xmlschema format: <str>"
Time.rfc2822 / .rfc822	Wdy, DD Mon YYYY HH:MM:SS ±HHMM	ArgumentError "not RFC 2822 compliant date: <str>"
Time.httpdate	RFC 1123 + obsolete RFC 850 + asctime	ArgumentError "not RFC 2616 compliant date: <str>"
Date/DateTime .iso8601 .xmlschema .rfc3339 .rfc2822 .rfc822 .httpdate	as above (Date drops the time; DateTime keeps wall clock + offset)	Date::Error "invalid date"

Date / DateTime rebuild through the go-ruby-date constructors, so the reform, JDN core and formatters stay authoritative; round-trips through the instance formatters (#iso8601/#rfc2822/#httpdate/…) hold.

Deferred

• Date.jisx0301 / DateTime.jisx0301 — the Japanese-era table (M/T/S/H/R base years) is unexported in go-ruby-date; a follow-up should expose or mirror it.
• A zone-less Time.iso8601 resolves to UTC here (deterministic) where MRI uses the machine local zone — the one inherently non-deterministic case.

Verification

• go test ./... green; whole-package coverage gate shows only the 10 pre-existing tolerated partials — every new function is 100% incl. error branches.
• gofmt + go vet ./internal/vm/ clean.
• ruby/spec ratchet: 12382 passing (files loaded 2103) — FLOOR 12351 → 12352.

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