Add opt-in Web API foundation: Options.WebApi, DOMException and console

[lahma]

This is the first PR of a planned series that gives Jint an opt-in, WHATWG-faithful set of web platform APIs — console, timers, TextEncoder, URL, AbortController and ultimately fetch — guided by WinterTC's Minimum Common Web Platform API. Users have asked for this since 2017 (#442, discussion #1253), and every embedder that wanted logging or HTTP has had to hand-roll its own shims.

Three ground rules hold for the whole series, and this PR lays them down:

• BCL only. No new package dependencies, which is why the surface is uniformly #if NET8_0_OR_GREATER (the same shape as the Float16 APIs and the System.Text.Json interop): on net462/netstandard2.x the options and types simply do not exist.
• Opt-in, and nothing else changes. A default engine is byte-for-byte the engine it was before: no globals, no construction cost (the options group itself is allocated on first touch), no behaviour change. UseWebApis() enables the non-network set; fetch, when it lands, will always require its own explicit UseFetch(...).
• The host wins. Globals are installed as lazy descriptors from Options.Apply, after the host's own configuration, and the install is non-clobbering — a console the host registered itself is left exactly as the host left it (checked with a probe, so a host's lazy global is not even materialized by the look). Only the principal realm is touched, so a ShadowRealm gets none of this.

What's in this PR

• Options.WebApi + [Flags] WebApiFeatures + UseWebApis/UseConsole extensions, and WebApiRegistration — the plumbing every later feature rides on. Flags are declared only once the feature behind them exists; the bit layout is reserved up front.
• DOMException per WebIDL: prototype chained to %Error.prototype% (so instanceof Error holds and stack works), name/message/code as brand-checked prototype accessors, the 25 legacy constants on both the interface object and the prototype. It has no feature flag — it is how every web API reports failure, so it exists whenever any of them does.
• console per the Console Standard: the log family, assert, trace, dir, group, count and time, with the %s/%d/%i/%f/%o/%O/%c/%% formatter, group indentation, and a bounded, cycle-safe, getter-free object inspection. Output goes to a host-supplied ConsoleSink (one finished line per record); the default sink discards everything, so enabling the feature never surprises a server host with stdout writes.
• Jint.Repl adopts UseConsole(Console.Out) and drops its hand-rolled JsConsole — first dogfooding consumer.
• One core fix the work surfaced: Error.prototype.toString passed %Error.prototype% as the receiver for its name/message reads where spec steps 3 and 5 say Get(O, …). Unobservable for ordinary errors (data properties); wrong for any error whose attributes are prototype accessors that brand-check their receiver, which DOMException's are. Full test262 stays green.
• README gains a "Web APIs (opt-in)" section with the roadmap table; AGENTS.md scopes the unconditional-registration rule to TC39 features and documents the new subtree's conventions.

Verification

• dotnet build -c Release — all five TFMs, 0 errors (the #if gate is proven by the downlevel targets compiling).
• Jint.Tests and Jint.Tests.PublicInterface green on net10.0 and net472, in both the default and the JINT_HOST_CONTRACT_VERIFICATION=1 configurations.
• Full test262 green (run because a core Error file was touched; the Error-filtered subset was run repeatedly).
• Pins included for: a default engine has no web globals; a host-registered global wins; ShadowRealm gets nothing; a snapshot restore returns the globals to their unmaterialized state.

The next PRs in the series, in dependency order: timers + queueMicrotask (the one event-loop change, benchmark-gated), then Event/AbortController, TextEncoder/TextDecoder, atob/btoa, structuredClone, crypto, performance, WHATWG URL, Blob/File/FormData, and finally fetch itself with bounded default policy (scheme allowlist, redirect-hop re-validation, response-size cap, timeout) and a threat-model entry.

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