Engine-owned host environment

Date: 2026-07-14 Area: engine Issue: #881

JavaScript-observable time, time zone, and randomness flow through one TGocciaHostEnvironment owned by each engine. Its small interface hides the system and injected clock/RNG adapters used by Math.random, Date.now, Temporal.Now, implicit Intl.DateTimeFormat time, and performance. Infrastructure timing remains on TimingUtils, so execution timeouts, profiling, garbage collection, benchmark measurement, and Test262 agent coordination continue advancing in deterministic mode.

The --deterministic CLI flag and "deterministic": true config key install one fixed profile: Unix epoch and monotonic time 0, time zone UTC, and a portable SplitMix64 stream seeded with unsigned 64-bit 0. Child realms share the clock and derive stable, distinct RNG streams. Hosts can instead inject IGocciaHostClock and IGocciaHostRandom adapters through Engine.HostEnvironment; configuration must happen before execution and before runtime extensions such as performance attach.

GocciaScriptLoader --host-environment=<module> provides the same injection seam to JavaScript callers. The module exports callable epochNanoseconds, monotonicNanoseconds, timeZoneIdentifier, and random providers and is evaluated before the loader runtime profile attaches, so performance captures the injected origin. Default system providers preserve each consumer's prior time-zone fallback; an explicitly configured provider overrides it.

This keeps the state per engine rather than global, follows per-engine realm isolation, and gives callers one seam for all host-controlled nondeterminism. It implements ECMA-262 2026 Math.random (§21.3.2.28, sec-math.random) and Date.now (§21.4.3.1, sec-date.now) against snapshot 0248456c758431e4bb8e5d26333ff1865123c9cd; Temporal follows HostSystemUTCEpochNanoseconds and SystemTimeZoneIdentifier from the current Temporal proposal.