Move the JavaScript frontend onto unibind, and ship a SpiderMonkey build

[ResurrectedTrader]

Moves the JavaScript frontend off V8's API onto unibind (ub::), a V8-shaped embedding API over V8 and SpiderMonkey. The frontend compiles once against unibind's headers - no engine header anywhere in the tree - and the engine is chosen when a DLL project links it:

DLL	Project	Links	Output
V8 (unchanged for users)	d2bs-v8.vcxproj	unibind_backend_v8.lib + V8 15.6.8	Release\js-v8-lod114d\d2bs.dll, released as d2bs.dll
SpiderMonkey (new)	d2bs-sm.vcxproj	unibind_backend_spidermonkey.lib + SpiderMonkey 153.3.0esr	Release\js-sm-lod114d\d2bs.dll, released as d2bs-spidermonkey.zip

What changed

The renames (#47, #48) and the return to plain V8 (#49) landed first, so this diff is the migration alone, and it keeps main's shape with unibind types in place of V8's:

• Bindings are declared through api::ClassBase over ub::Class (api/core/Class.h), and the api/core/{Convert,Extract,Error,Function}.h helpers work in unibind values. Class properties are declared on the instance template, so they stay own properties of each instance, as on main.
• Events are main's events with unibind types: MakeArgs(const ub::Context&), Execute(context, fns) with the handler loop inline, timers holding a ub::Global<ub::Function>, drawable click/hover handlers resolved on the script thread, and the same lenient broadcast serialisation.
• Engine::GetPlatform() owns the ub::Platform, with one fault handler for both engines (fatal errors, V8 DCHECKs and SpiderMonkey MOZ_CRASH all arrive as EngineFault::Fatal). The console reads ub::HeapStatistics and ub::Platform::BackendName() / BackendVersion() directly.
• Script owns its ScriptInspector directly, as on main. ub::Inspector is null on SpiderMonkey, which has none; the server is started, and the console's debugging settings shown, only where ub::Inspector::Supported().
• Teardown collects until every wrapper is finalized and reports what is still referenced before the isolate goes, and waits for other holders of the isolate without spinning.
• FetchUnibind and FetchSpiderMonkey sit beside FetchV8, through one scripts/fetch_archive.ps1 (unibind's archive is checked against its published SHA-256). The backend no longer compiles against V8. CI and the release build and ship both DLLs.
• scripts/check_compile.ps1: compile one frontend file for errors without MSBuild.
• On main's layout (lint: discover projects instead of listing them #50-Land #53 and #54 on main: namespaces + flattened layout #55): the frontend is src/runtime in d2bs::runtime::* namespaces, both DLL projects live under src/dlls/ (js-v8-lod114d, js-sm-lod114d), lint.ps1 finds a project's compile database by its TargetName (both DLL projects build d2bs.dll), and gen_enum_names.py parses against unibind's headers instead of V8's.

JS-visible differences

The extracted API surface (api.json: every class, member, readonly flag, constructability, global, constant, me member and event) is identical to v2.1.6's; only two descriptions now say "the engine" where they said "V8". Behaviour that differs:

• Class properties are own properties of each instance as before, but real accessors rather than V8's data-looking native properties: Object.getOwnPropertyDescriptor shows get/set, assigning to a read-only one throws in strict code (it used to silently replace the value), and reading one through Object.create(unit) throws Illegal invocation.
• A getter or method called on a receiver of the wrong class throws TypeError: Illegal invocation instead of crashing.
• A drawable stays readable after remove() (it used to read undefined); writes do nothing.
• getDialogLines() handlers are not constructable (new handler() throws).
• Promise continuations run at the event loop's pump points (after the top level, after main(), inside delay()), not whenever a native call returns.
• An HTTP body may again be any ArrayBufferView; sendPacket takes an ArrayBuffer or typed array as before.
• Error reports with no known line print :0: rather than :-1:; syntax errors now report file, line and the offending source line.
• The console's Settings row shows the engine's name and version ("V8 15.6.8", "SpiderMonkey 153.3.0esr") where main shows "V8 version".

Verification

• build.ps1 Release (both DLLs, no linker warnings), build.ps1 Release -Platform x64, build.ps1 test (157/157), check-format, lint (126/126) - all clean, with unibind 0.6.3 fetched from its release. extract_api.py: API surface identical to v2.1.6.
• Run in the game on the V8 build (kolbot SoloPlay, the console, DevTools pause and breakpoints). That run caught regressions fixed here: class properties had moved onto the prototype, so kolbot's copyObj (for...in + hasOwnProperty) copied nothing; and script teardown spun a core for ever when DevTools was enabled. unibind 0.6.2 / 0.6.3 also made wrapping an object with many members (copyUnit, getUnit) fast on SpiderMonkey. The SpiderMonkey DLL still needs its live run, and this rebuilt branch a fresh one on V8.

Licensing and analytics

• THIRD-PARTY-NOTICES.txt (every component's own licence text, and the SpiderMonkey source location MPL-2.0 asks for) ships with each release: beside d2bs.dll and inside d2bs-spidermonkey.zip.
• Analytics' session_start records engine / engineVersion; the inspector feature tag counts only where the engine has a debugger (docs/analytics.md).

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