Detect terminal color capabilities

REQUIREMENTS.md

@@ -31,6 +31,8 @@ Rows are redgate TSV requirements: `ring<TAB>id<TAB>summary [tag]`.
 0	001	Narrow terminals SHALL use transient panels instead of permanent desktop splits. [mobile]
 1	002	Files, git, diagnostics, LSP, build, terminal, and Pi surfaces SHALL share one panel navigation model. [panels]
 0	003	When the cursor moves beyond the visible horizontal or vertical viewport, the editor SHALL scroll enough to keep the cursor cell visible. [mobile]
+0	004	The default visual theme SHALL preserve semantic color identity for syntax, selections, diagnostics, job/tool status, hover/signature/context surfaces, and panel chrome on mobile SSH terminals. [mobile]
+0	005	Terminal color adaptation SHALL choose truecolor, 256-color, 16-color, or mono output from local terminal environment signals with no runtime config files. [mobile]
 
 ## coding
 

src/theme.zig

@@ -39,6 +39,25 @@ pub const ColorMode = enum {
     mono,
 };
 
+pub const Background = enum {
+    dark,
+    light,
+    unknown,
+};
+
+pub const TerminalEnv = struct {
+    term: ?[]const u8 = null,
+    colorterm: ?[]const u8 = null,
+    no_color: ?[]const u8 = null,
+    mim_color: ?[]const u8 = null,
+    colorfgbg: ?[]const u8 = null,
+};
+
+pub const TerminalCapabilities = struct {
+    color_mode: ColorMode,
+    background: Background,
+};
+
 pub const SgrKind = enum { fg, bg };
 
 pub const AdaptedColor = union(enum) {
@@ -98,6 +117,61 @@ pub fn rgb(r: u8, g: u8, b: u8) Color {
     return .{ .r = r, .g = g, .b = b };
 }
 
+pub fn detectTerminal(env: TerminalEnv) TerminalCapabilities {
+    return .{
+        .color_mode = detectColorMode(env),
+        .background = detectBackground(env),
+    };
+}
+
+pub fn detectColorMode(env: TerminalEnv) ColorMode {
+    if (env.no_color != null) return .mono;
+    if (env.mim_color) |forced| {
+        if (std.ascii.eqlIgnoreCase(forced, "mono")) return .mono;
+        if (std.ascii.eqlIgnoreCase(forced, "16")) return .color16;
+        if (std.ascii.eqlIgnoreCase(forced, "256")) return .color256;
+        if (std.ascii.eqlIgnoreCase(forced, "truecolor") or std.ascii.eqlIgnoreCase(forced, "24bit")) return .truecolor;
+    }
+    if (env.term) |term| {
+        if (std.mem.eql(u8, term, "dumb")) return .mono;
+    }
+    if (env.colorterm) |colorterm| {
+        if (containsIgnoreCase(colorterm, "truecolor") or containsIgnoreCase(colorterm, "24bit")) return .truecolor;
+    }
+    if (env.term) |term| {
+        if (containsIgnoreCase(term, "truecolor") or containsIgnoreCase(term, "24bit")) return .truecolor;
+        if (containsIgnoreCase(term, "256color")) return .color256;
+        if (containsIgnoreCase(term, "color")) return .color16;
+    }
+    return .color16;
+}
+
+pub fn detectBackground(env: TerminalEnv) Background {
+    const value = env.colorfgbg orelse return .unknown;
+    var fields = std.mem.splitScalar(u8, value, ';');
+    var last: ?[]const u8 = null;
+    while (fields.next()) |field| last = field;
+    const bg = last orelse return .unknown;
+    const code = std.fmt.parseInt(u8, bg, 10) catch return .unknown;
+    return if (code >= 0 and code <= 6) .dark else .light;
+}
+
+pub fn fromProcessEnv() TerminalCapabilities {
+    return detectTerminal(.{
+        .term = std.posix.getenv("TERM"),
+        .colorterm = std.posix.getenv("COLORTERM"),
+        .no_color = std.posix.getenv("NO_COLOR"),
+        .mim_color = std.posix.getenv("MIM_COLOR"),
+        .colorfgbg = std.posix.getenv("COLORFGBG"),
+    });
+}
+
+pub fn hasAccessibleContrast(style: Style, fallback_bg: Color, minimum_ratio: f32) bool {
+    const fg = style.fg orelse return true;
+    const bg = style.bg orelse fallback_bg;
+    return contrastRatio(fg, bg) >= minimum_ratio;
+}
+
 pub fn adaptColor(color: Color, mode: ColorMode) AdaptedColor {
     return switch (mode) {
         .truecolor => .{ .rgb = color },
@@ -156,6 +230,16 @@ fn sgrBase(kind: SgrKind) u8 {
     };
 }
 
+fn containsIgnoreCase(haystack: []const u8, needle: []const u8) bool {
+    if (needle.len == 0) return true;
+    if (needle.len > haystack.len) return false;
+    var i: usize = 0;
+    while (i + needle.len <= haystack.len) : (i += 1) {
+        if (std.ascii.eqlIgnoreCase(haystack[i .. i + needle.len], needle)) return true;
+    }
+    return false;
+}
+
 fn nearest16(color: Color) u8 {
     return nearestInTable(color, ansi16[0..]);
 }
@@ -291,3 +375,32 @@ test "regular: 256 and 16 color SGR use indexed form" {
     defer std.testing.allocator.free(rendered16);
     try std.testing.expect(std.mem.startsWith(u8, rendered16, "\x1b[48;5;"));
 }
+
+test "regular: terminal color-mode detection follows safe precedence" {
+    try std.testing.expectEqual(ColorMode.mono, detectColorMode(.{ .no_color = "1", .colorterm = "truecolor", .term = "xterm-256color" }));
+    try std.testing.expectEqual(ColorMode.mono, detectColorMode(.{ .term = "dumb" }));
+    try std.testing.expectEqual(ColorMode.truecolor, detectColorMode(.{ .colorterm = "truecolor", .term = "xterm-256color" }));
+    try std.testing.expectEqual(ColorMode.truecolor, detectColorMode(.{ .mim_color = "24bit", .term = "screen" }));
+    try std.testing.expectEqual(ColorMode.color256, detectColorMode(.{ .term = "xterm-256color" }));
+    try std.testing.expectEqual(ColorMode.color16, detectColorMode(.{ .term = "xterm-color" }));
+    try std.testing.expectEqual(ColorMode.color16, detectColorMode(.{}));
+}
+
+test "regular: terminal background detection uses COLORFGBG final field" {
+    try std.testing.expectEqual(Background.dark, detectBackground(.{ .colorfgbg = "15;0" }));
+    try std.testing.expectEqual(Background.light, detectBackground(.{ .colorfgbg = "0;15" }));
+    try std.testing.expectEqual(Background.unknown, detectBackground(.{ .colorfgbg = "broken" }));
+    try std.testing.expectEqual(Background.unknown, detectBackground(.{}));
+}
+
+test "regular: terminal capability detector combines color and background" {
+    const caps = detectTerminal(.{ .term = "xterm-256color", .colorfgbg = "15;0" });
+    try std.testing.expectEqual(ColorMode.color256, caps.color_mode);
+    try std.testing.expectEqual(Background.dark, caps.background);
+}
+
+test "regular: accessibility guardrail checks style contrast" {
+    try std.testing.expect(hasAccessibleContrast(newed_night.normal, newed_night.normal.bg.?, 7.0));
+    try std.testing.expect(!hasAccessibleContrast(.{ .fg = rgb(30, 30, 35), .bg = rgb(25, 22, 33) }, newed_night.normal.bg.?, 4.5));
+    try std.testing.expect(hasAccessibleContrast(.{ .bg = rgb(25, 22, 33) }, newed_night.normal.bg.?, 4.5));
+}