osd: parse custom message FIFO by newline and render multi-line popups

[evanofficial]

Fixes #98.

/run/pixelpilot.msg was read once per second and whatever fit in a single non-blocking read() became one fact, so concurrent writers got glued together and a literal \n in the message rendered as a tofu square in the popup widget.

Changes

src/main.cpp — CustomMsgManager

• Reads now accumulate into a std::string buffer instead of being published verbatim.
• Each \n-terminated chunk in the buffer is emitted as its own osd.custom_message fact, so multiple messages written during a single 1-second tick no longer collide.
• If a writer never sends a newline, the buffer is flushed once it reaches MAX_MSG_LEN (512 bytes) so we cannot grow unboundedly.
• Before publishing, the message is unescaped: a literal \n becomes a real newline. This lets writers that cannot emit raw newlines (e.g. plain echo, shell printf without -e, some scripting hosts) still produce multi-line messages.

src/osd.cpp — PopupWidget::draw

• The widget uses cairo_show_text, which doesn't honour \n — embedded newlines used to render as a tofu glyph. Each message is now split on \n and drawn as separate lines.
• The background box is sized to the longest line and the total stacked height, so the box still wraps the whole message rather than just the first line.

Testing

• Verified the parser logic (escape unescape, buffer split across reads, multi-message split, escaped-newline -> split) locally with a small standalone harness.
• Did not run a full build on this host — the project cross-compiles for Rockchip ARM and requires librockchip-mpp-dev, libdrm-dev, etc., which I don't have set up locally. The changed code only uses <string> / <vector> and existing cairo calls that were already in the file, so it should compile on the target.

Notes

• MAX_MSG_LEN is 512 (well above the previous 119-byte single-read cap, but small enough to stay safe). Happy to bump it if you'd prefer a different ceiling.
• The split-then-render approach in the popup keeps line spacing at 2 px, matching the existing inter-message spacing.