cli: use the terminal palette for low-confidence groups

[shivamtiwari3]

Fixes #1243.

Summary

The Rust CLI's fallback color for content type groups without an explicit mapping (including the common "text" and "unknown", plus "binary", "database", "font", "geometry", "gis", "model", "scientific", "inode" and "undefined") was a fixed light gray, #cccccc:

_ => result.bold().truecolor(0xcc, 0xcc, 0xcc),

On a light-background terminal this is barely visible, which is what #1243 reports ("The very light gray used by low-quality detection reports works very poorly there").

Fix

Use the terminal palette for the fallback instead of a fixed truecolor:

_ => result.bold().bright_black(),

bright_black maps to the terminal palette (ANSI bright black), so it renders as a dark gray on light backgrounds and as a light gray on dark backgrounds. It adapts to the user's theme, so no --light-mode flag or extra configuration is needed. This follows the preference already expressed on #1243 by @ia0 that using the palette is the safest choice.

The eight highlighted groups keep their deliberate Tailwind truecolors; only the low-confidence fallback changes.

Testing

• rust/cli/test.sh: added a regression test that runs the CLI with --colors on a plain .txt file (group "text"), asserts the output contains the palette code (\x1b[1;90m) and does not contain the old #cccccc truecolor (\x1b[1;38;2;204;204;204m).
• cargo build, cargo clippy -- --deny=warnings and cargo fmt -- --check are clean.
• Manually verified: magika --colors text.txt now emits \x1b[1;90m...\x1b[0m.

Notes

This is the Rust CLI counterpart of the now-closed #1317 (which targeted the removed Python fallback client). Opened as requested by @ebursztein on #1317, where #1243 was left open for the Rust CLI. Happy to switch to a --light-mode flag or a --color-file option (both suggested on #1243) if you prefer that direction.

[shivamtiwari3]

@ia0 since you own the Rust code, could you take a look when you have a moment? It's a small change in rust/cli/src/main.rs (one line) plus a test in rust/cli/test.sh.

The Rust CI job hasn't run yet — it may be waiting on a maintainer to approve workflows for this fork PR. Locally I get clean cargo build, cargo clippy -- -D warnings, and cargo fmt --check.

Heads up that #1531 also touches rust/cli/src/main.rs; if it lands first I'll rebase. I'm also happy to switch this to a --light-mode flag or --color-file option if that's the direction you'd prefer over the palette.