feat(vm): Time/Date standard formatters — Time#iso8601/xmlschema/rfc2822/httpdate

[tannevaled]

Horodate: 2026-08-09 22:54 CEST

What

Implements the deferred require "time" instance formatters on Time, built on the VM's existing strftime engine (Date/DateTime already shipped theirs in #340):

• Time#iso8601(fraction_digits=0) and its alias Time#xmlschema — %Y-%m-%dT%H:%M:%S, optional .NNN fractional part (n truncated digits), then Z for a UTC instant or %:z (+HH:MM) for a fixed-offset one.
• Time#rfc2822 / Time#rfc822 — %a, %d %b %Y %H:%M:%S %z, where a UTC instant reports RFC 2822's -0000 unknown-local-zone marker (MRI quirk), a fixed offset its +HHMM.
• Time#httpdate — the instant converted to GMT as %a, %d %b %Y %H:%M:%S GMT.

Every output was verified byte-for-byte against ruby 4.0.5 (ruby -rtime -e …), including the UTC-Z vs +00:00 distinction, the -0000 vs +0000 rfc2822 distinction, fraction-digit variants, and the httpdate GMT reconversion.

Deferred (noted, not implemented)

• The class-method parsers Time.iso8601 / Time.xmlschema / Time.rfc2822 / Time.httpdate. MRI's are strict (each rejects the other formats with ArgumentError), so they need real format-specific parsers rather than routing through the existing lenient Time.parse (gotime.ParseAny) — out of scope for a clean, 100%-coverable change. Time.parse / Time.strptime are untouched.
• Time#rfc3339 — does not exist in MRI (Time.respond_to?(:rfc3339) → false; NoMethodError on instances), so nothing to add. Date/DateTime already have #rfc3339.

Verification

• New Go table cases in internal/vm/time_test.go asserting literal MRI outputs on fixed deterministic instants (UTC + +02:00 + +00:00, fraction variants). go test ./internal/vm -run TestTime green.
• Full GOWORK=off go test ./... — exit 0, no regressions.
• CI-exact whole-package coverage gate (grep -vE '100.0%' minus the tolerated pre-existing partials) — empty; iso8601Str / rfc2822Str / httpdateStr each 100.0%.
• gofmt + go vet ./internal/vm/ clean (pre-existing builtins.go delta untouched).
• ruby/spec ratchet: 12381 passing (files loaded 2103), ≥ FLOOR; FLOOR bumped 12346 → 12351 (N−30).

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