build: an app lists the fizzy plugins it links

[foxnne]

Part of fizzyedit/plans#13 (step 1).

What changes

An app built on fizzy chooses which of fizzy's plugins it links. fizzy.plugins.{workbench,text,image,archive,markdown,devtools} are entries in the list fizzy.buildApp takes, beside the app's own, and fizzy.plugins.all is fizzy's own set:

try fizzy.buildApp(fizzy_dep, &.{
    fizzy.plugins.workbench,
    fizzy.plugins.text,
    .{ .name = "graph", .module = graph_dep.module("plugin") },
});

Fizzy's own entries have no module: build/plugins.zig's link builds each one for every executable (native, packaged, web), wired to that executable's framework, and re-homes the app's modules the way the native and web builds each did on their own before. Fizzy's build passes fizzy.plugins.all through the same path, so the exe and the web build no longer keep separate copies of the list. The test build keeps its own list, because it tests fizzy itself. A plugin that isn't listed doesn't register, and fizzy's dylib for it (text, markdown, image) is neither built nor installed.

Two plugins are always built, because fizzy's own code imports them as libraries. workbench must be listed: the editor still holds its state (editor.workbench, the dylib's arg_c), and a list without it fails the build with a message naming the plan. markdown is always compiled, because the store draws READMEs with it, but it only registers as a plugin when listed.

Transition: fizzy's CI builds example-app's main, and example-app pins fizzy by commit, so neither repo can make the change first. Until example-app lists fizzy's plugins, a buildApp list that names none of them gets all of them first, as before. That fallback is three lines in buildApp, and step 2 removes it. fizzyedit/example-app#6 lists them explicitly (its minimal shape links only workbench and text), and it repins to this once merged.

Also:

• Entry.zig drops an unused @import("text").
• The postInit comment no longer says the bundled plugins don't ship as dylibs. They do, and it now says why.
• The root AGENTS.md describes the list.

Overlaps in files only with the Zig 0.17 drafts (#437, #438), which touch the whole build; this change touches no build step.

SDK impact

• None

Verified

• macOS: zig build, check-web, test, test-integration, test-sdk-version, check-fmt. The four built-in dylibs are still installed to zig-out/<target>/plugins/. scripts/check-examples.sh passes against example-app with explicit lists (its minimal shape links only workbench and text), and against example-app's main through the fallback.
• Windows:
• Linux: CI
• Web: check-web

Follow-ups

• Step 2 (plans#13): a link-only build, and removing the fallback.
• workbench becomes optional once its state moves from the Editor into the plugin.

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