[fizzy-macos]

In plain words

On a phone, pressing Enter in the chat's message box now starts a new line instead of sending, because a phone's Send button is what sends. On a computer Enter still sends and Shift+Enter adds a line. The box also grows taller before it starts scrolling, so longer messages stay readable while you write them.

What you will notice

On a phone or tablet build, Enter adds a line. On desktop nothing changes except the box growing to eight lines (was six) before it scrolls.

How this serves fizzy

Goal: phone typing (mobile): the same chat composes comfortably on a phone as on a desktop.

Goal

Port the composer rules from fizzy's #910 into the chat's message box (fizzyedit/work#530).

What changed and why

• src/composer.zig (std-only): enter(shift, mobile) and max_lines = 8, with unit tests, wired into View.drawInput.
• build.zig: the new tests run under zig build test.

How it was verified

• zig fmt --check build.zig plugin.zig src: clean
• zig build test: passes (includes the new composer tests; the Enter test fails without the change as the file is new)
• zig build (into a temporary FIZZY_PROFILE): passes

Not done / follow-ups

• Mobile detection: SDK 0.2.32 does not expose the app's form factor to plugins (no core.form_factor), so composer.isMobile() uses the build target (iOS/Android). A phone's web browser or a narrow desktop layout still counts as desktop; needs the SDK to expose it (the issue says no new SDK types).
• Fences: not ported as a Segments splitter. Message text already goes through the markdown service, which renders fenced code, so a splitter would be dead code here.
• chat.sample client: not added (the repo has no scripted client; chat/no-sample removed it); separate piece.
• The phone-shaped tape/screenshot was not made: no windows on this machine; left to the QA loop.

Part of fizzyedit/work#530