Opt-in batch serialisation for shared-main-thread runners; make jsaddle-wkwebview TH-loadable; aeson <2.4

[hamishmack]

Three independent groups of changes, one commit each (or a small run of commits) so they can be reviewed separately.

1. runJavaScriptWithSerializer — opt-in batch serialisation (5df2029)

runJavaScript gains an optional serialiser (Maybe (MVar ())) that brackets the batch round-trip (sendBatch..takeResult); runJavaScript = runJavaScriptWithSerializer Nothing, so nothing changes for existing runners. jsaddle-warp, -terminal, -clib and the null runner keep running their contexts fully in parallel — each browser client is its own context on its own transport, and serialising across them would let one slow client stall the rest.

jsaddle-wkwebview passes Just a module-global lock shared by all windows, because the contended resource is shared: every window dispatches its JS onto the one Cocoa main queue, and a synchronous window.prompt round-trip blocks that queue, so two windows driving JS concurrently can wedge each other.

The lock is safe against jsaddle's sync-callback protocol: a prompt handler returns the pre-set lastAsyncBatch without needing the batch thread, so blocking the batch thread never stalls an in-flight synchronous round-trip. It is also the only sendBatch call site in the function, so there is no unbracketed path.

2. aeson < 2.4 (c8f04af, ead142a)

Allows aeson 2.3.x for jsaddle, jsaddle-wkwebview, jsaddle-webkitgtk and jsaddle-webview2.

⚠️ Not done here, and worth a decision: jsaddle-warp, jsaddle-clib and jsaddle-webkit2gtk still cap at <2.3, so a project combining those with aeson 2.3 still fails to solve. I left them alone rather than widen a bound I haven't built against — say the word and I'll bump and test them in this PR.

3. One Cocoa ObjC translation unit, so dependents' TH splices can load the library (c4d992b, cdf4bee, e0d6d03, c583144)

Two separate ways this package broke Template Haskell in dependents under a statically linked GHC:

• Archive-member basename collision. Cabal 3.12+ compiles foreign sources into the same object tree as Haskell modules, ar stores members by basename, and GHC 9.14's in-process TH loader resolves members by name — so cbits/WKWebView.m and the Language.Javascript.JSaddle.WKWebView module both producing WKWebView.o made any TH splice that loads this package fail with a spurious duplicate definition for symbol _openApp (hit building leksah's gi-gtk splices). Fixed by renaming the cbits file to WKWebView-cbits.m.
• Duplicate ObjC metadata across archive members. GHC's runtime linker treats the protocol/class metadata every ObjC object file emits (e.g. __OBJC_LABEL_PROTOCOL_$_NSObject) as strong duplicate definitions — the system linker coalesces them — so a library with more than one ObjC archive member won't load. The Cocoa sources are now combined into a single translation unit, cbits-cocoa/WKWebView-AppDelegate.m, which #includes both; the base cbits source moved into each conditional branch so it is never compiled twice. Combining them puts the whole TU into clang's nullability-audit mode (AppDelegate.m uses _Nonnull), which then demands annotations on every pointer in the other file, so the composite silences the two completeness diagnostics — both files still compile cleanly standalone.
• sdist. Because the two combined files are now #included rather than compiled directly, they left every cxx-sources list and cabal stopped shipping them, so the composite's #include failed with file not found from a tarball. They are listed in extra-source-files now.

The iOS/UIKit branch and the include-app-delegate: False branch keep compiling cbits/WKWebView-cbits.m directly.

Testing

• jsaddle and jsaddle-wkwebview build clean on macOS/aarch64, GHC 9.14.
• cabal sdist jsaddle-wkwebview now contains cbits/WKWebView-cbits.m, cbits-cocoa/AppDelegate.m and cbits-cocoa/WKWebView-AppDelegate.m — i.e. the composite #include resolves from a released tarball.
• All of this has been in daily use in leksah (multi-window WKWebView, gi-gtk TH splices, GHC 9.14) — the serialiser lock and both TH-loader fixes were found there.
• Not covered by CI: this repo's Haskell-CI is Linux-only, so it exercises group 1 (jsaddle) and group 2, but never compiles the wkwebview ObjC.

[Copilot]

Pull request overview

This PR introduces an opt-in mechanism to serialize JSaddle batch round-trips for runners that share a single UI/main-thread transport (notably WKWebView), widens aeson upper bounds to allow 2.3.x, and adjusts jsaddle-wkwebview’s Objective-C build layout to avoid Template Haskell runtime linker failures under statically linked GHC.

Changes:

• Add runJavaScriptWithSerializer and route runJavaScript through it to optionally serialize each sendBatch..takeResult round-trip.
• Widen aeson upper bounds from <2.3 to <2.4 in several packages.
• Rework jsaddle-wkwebview ObjC sources: rename cbits file to avoid archive-member basename collisions, and compile Cocoa sources as a single translation unit; ensure #included sources are shipped via extra-source-files.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
jsaddle/src/Language/Javascript/JSaddle/Run.hs	Adds runJavaScriptWithSerializer and brackets the batch round-trip with an optional MVar serializer.
jsaddle/jsaddle.cabal	Widens aeson upper bound to <2.4.
jsaddle-wkwebview/src/Language/Javascript/JSaddle/WKWebView/Internal.hs	Uses a module-global lock and calls runJavaScriptWithSerializer (Just lock) to serialize batch round-trips across windows.
jsaddle-wkwebview/jsaddle-wkwebview.cabal	Switches Cocoa build to a single ObjC TU and adds extra-source-files for #included sources; widens aeson bound.
jsaddle-wkwebview/cbits/WKWebView-cbits.m	Introduces renamed WKWebView cbits implementation to avoid object-name collisions.
jsaddle-wkwebview/cbits-cocoa/WKWebView-AppDelegate.m	Adds composite Cocoa translation unit including the cbits and AppDelegate sources.
jsaddle-webview2/jsaddle-webview2.cabal	Widens aeson upper bound to <2.4.
jsaddle-webkitgtk/jsaddle-webkitgtk.cabal	Widens aeson upper bound to <2.4.

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