Add a deterministic execution mode (injectable clock + seeded RNG)

[frostney]

Summary

Add a deterministic execution mode so a given source + config + host produces repeatable output: a host-injectable frozen clock and a seeded portable PRNG.

Why

Reproducible runs are core to the sandbox-for-AI vision (reproducible agent runs and reproducible test262), but every non-deterministic source reads a global OS/FPC primitive directly with no injection seam: Math.random calls FPC Random (Goccia.Builtins.Math.pas:479), the clock is TimingUtils.GetEpochNanoseconds / GetNanoseconds (raw clock_gettime / QueryPerformanceCounter), and Temporal.Now / Date.now / performance.now read those. Object/Map/Set iteration order is already deterministic, so no change is needed there.

Current behavior

• Math.random -> FPC Random, never seeded, no seam.
• Wall clock GetEpochNanoseconds feeds Temporal.Now and (via shim) Date.now / new Date(); monotonic GetNanoseconds feeds performance.now.
• Nothing accepts a seed or injectable clock. (GocciaBenchmarkRunner --profile-deterministic is unrelated — a naming-collision risk.)

Expected behavior

Decided (grill): an engine-wide --deterministic CLI option (plus config key) that:

• Injects a single synthetic clock at the TimingUtils seam, frozen at a fixed epoch — making Temporal.Now and Date deterministic together (shared GetEpochNanoseconds); performance.now (separate GetNanoseconds) is frozen the same way and timeOrigin is pinned.
• Seeds a portable PRNG (e.g. splitmix64 / PCG, not FPC Random) for Math.random, so output is stable across platforms and FPC versions.
• Pins the system timezone to UTC (Temporal.Now.timeZoneId / Date read host TZ today).

Scope notes

• Cleanest seam: a clock provider behind TimingUtils.GetEpochNanoseconds + GetNanoseconds (both in source/shared, used by runtime and tooling). Avoid the --profile-deterministic name collision.
• Warrants an ADR (cross-cutting seam, hard to reverse). Parent Reproducible and observable agent runs (deterministic mode + audit log) #828.