Add Debian 12 (GTK 4.8 / libadwaita 1.2) build compatibility

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What

Adds a compatibility layer so séance builds and runs on Debian 12 "bookworm", which ships GTK 4.8.3 and libadwaita 1.2.2 — older than séance's current target (GTK 4.12+, libadwaita 1.4/1.5+). Useful for Debian 12 and other LTS distros that haven't moved to libadwaita 1.4+ yet. Tested on arm64 (aarch64); the substitutions apply equally to amd64.

Why

A clean zig build on Debian 12 fails with ~50 missing-symbol errors across the UI code, because several GTK/libadwaita calls don't exist in the older libraries.

How

Each newer API is replaced with a behaviourally equivalent one that already exists in GTK 4.8 / libadwaita 1.2 (source-level only, no new runtime dependencies):

Upstream API (min version)	Debian 12 replacement
gtk_gl_area_set_allowed_apis (GTK 4.12)	gtk_gl_area_set_use_es(area, FALSE) — keeps the desktop-GL request
gtk_css_provider_load_from_string (GTK 4.12)	gtk_css_provider_load_from_data(p, data, -1)
gtk_search_entry_set_placeholder_text (GTK 4.10)	placeholder set on the entry's GtkText editable delegate
GtkFontDialogButton (GTK 4.10)	GtkFontButton (via GtkFontChooser font-desc)
AdwAlertDialog (libadwaita 1.5)	AdwMessageDialog (1.2)
AdwDialog (libadwaita 1.5)	AdwWindow + GtkWindow (modal, transient-for, present)
AdwToolbarView (libadwaita 1.4)	vertical GtkBox (header on top, content below)
AdwBanner (libadwaita 1.4)	GtkRevealer + label + optional action button
AdwSpinRow (libadwaita 1.4)	AdwActionRow + GtkSpinButton
AdwSwitchRow (libadwaita 1.4)	AdwActionRow + GtkSwitch
adw_combo_row_set_enable_search (libadwaita 1.4)	omitted (search box is optional)

Also adds packaging/debian12/:

• install.sh — installs build deps, fetches Zig 0.15.2 (not packaged in Debian 12), builds, installs.
• README.md — documents the substitutions, dependencies, and the GPU note below.

Testing

On Debian 12 arm64:

• ✅ Builds cleanly from source (libghostty + séance).
• ✅ Launches and stays running; seance ctl ping → pong; seance ctl tree / identify work.
• ✅ OpenGL context creation succeeds.

Caveats / help wanted:

• My test box is a VM with virtio-gpu and no accelerated GL, so I had to force Mesa's software renderer (LIBGL_ALWAYS_SOFTWARE=1) to get a GL context. The gtk_gl_area_set_use_es(FALSE) substitution preserves the original desktop-GL request, but I'd appreciate confirmation on hardware-GL arm64.
• Because the panes only initialise once the compositor maps/paints the window, I couldn't fully exercise interactive terminal rendering in my headless session — would value a sanity check on a real desktop.
• There's a non-fatal Adwaita-CRITICAL ... adw_tab_view_close_page: assertion 'page_belongs_to_this_view' failed logged once at startup. It doesn't appear related to these substitutions (it's in tab-view setup), but flagging it in case it's meaningful.

Happy to adjust the approach (e.g. gate the older paths behind a libadwaita-version check / build option) if you'd prefer to keep the modern APIs as the default.

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🤖 Generated with Claude Code

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Update: I was able to confirm interactive pane rendering after all. Running against a painting display (an Xvfb framebuffer with Mesa software GL on the same arm64 box), the GtkGLArea renders and ghostty spawns the pane's shell — a live sh child + pty appear, the pane title tracks the shell prompt (debian@debian: ~), and the header bar, sidebar, and terminal all draw correctly. So the earlier "couldn't fully exercise rendering" caveat was just my headless session never compositing a frame, not a functional gap. The adw_tab_view_close_page startup warning is still emitted once but is non-fatal (pane works).