app: a link-only build, which loads no plugin at runtime

[foxnne]

Part of fizzyedit/plans#13 (step 2). Builds on #441.

What changes

An app built on fizzy can run with linked plugins only:

const fizzy_dep = b.dependency("fizzy", .{ …, .@"defer-app" = true, .@"dynamic-plugins" = false });

(or zig build -Ddynamic-plugins=false). In that build:

• No plugin dylib is built or installed. fizzy's four bundled ones are skipped, as are the packaged copies.
• Nothing loads at runtime. One comptime constant, App.loads_plugins, turns off:
  • the bundled-dylib probe in postInit
  • the user plugins directory scan (loadUserPlugins)
  • loading a plugin by id (the store's and settings' enable path)
  • a web page's ?plugin= request
• No store. PluginStore.register still sets the PluginManager (settings name plugins through it), then returns before the catalog fetch and the store's view. tick returns at once.

Every gate is comptime, as wasm's are, so a dynamic build checks nothing at runtime. Most other dynamic paths go quiet on their own when nothing is dlopened: the rebuilt-plugin watcher walks only the loaded libraries.

CI compiles the link-only build on Linux (-Ddynamic-plugins=false -Dno-emit, about 10 s locally), because its gates are comptime and the default build never compiles that branch.

SDK impact

• None

Verified

• macOS:
  • Build: zig build -Ddynamic-plugins=false installs only the executable, with no plugins/ directory.
  • Runtime: a text.dylib planted beside the executable and in the profile's user plugins directory is not mapped into the link-only process (vmmap). The default build, run with the same profile, maps all four of fizzy's dylibs.
  • Screenshots: the link-only window has no store icon in the rail, and the default build has it above the settings cog.
  • Gates on the default build: zig build, test (91/91 steps), test-integration (53/53 steps), check-tests-reached, test-sdk-version, check-fmt.
  • Web: check-web with and without the flag.
• Windows:
• Linux: CI (including the new link-only compile)
• Web: compiles link-only; not run in a browser

Not exercised: opening Settings in the link-only build. Its plugin rows read through the PluginManager, which is set as before.

Follow-ups

• Removing the buildApp fallback (a list naming none of fizzy's plugins gets all of them) waits until build: fizzy's plugins are listed; the minimal shape links two example-app#6 is on example-app's main, because fizzy's CI builds that main. It's a small PR after the repin.
• example-app: a link-only build in its CI and in check-examples.sh, after this merges, for the same ordering reason.

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