perf(display): cache terminal theme detection with 100ms timeout

[tusharmath]

Summary

Cache terminal theme detection with a 100ms timeout to avoid repeated blocking queries on every syntax-highlighted output.

Context

Terminal theme detection (terminal_colorsaurus::theme_mode) was called on every invocation of syntax highlighting and markdown rendering. This sends an escape-sequence query to the terminal and waits for a response, which is an I/O-bound operation that adds latency to every render cycle. In non-interactive environments (CI, piped output, some terminal emulators) the query can block for the full default timeout before falling back to dark mode, visibly slowing down output.

Changes

• Added a OnceLock<bool> / OnceLock<ThemeMode> process-wide cache in both forge_display and forge_markdown_stream so the terminal is queried at most once per process lifetime.
• Set an explicit 100 ms upper-bound timeout on the QueryOptions in both crates, replacing the library default (which can be several seconds).
• Added THEME_DETECT_TIMEOUT constants and doc-comments explaining the caching and fallback behaviour.

Key Implementation Details

std::sync::OnceLock provides lock-free reads after the first initialization, making subsequent calls to is_dark_theme() / detect_theme_mode() essentially free (a single atomic load). The 100 ms timeout is a safe upper bound: real terminals respond in < 10 ms, while environments that do not support the query will now fail-fast instead of stalling.

Testing

# Run display and markdown-stream crate tests
cargo insta test --accept -p forge_display -p forge_markdown_stream

# Observe that syntax-highlighted output is rendered without noticeable delay
# even when running in a piped/non-TTY context
cargo run -- "print hello world" 2>&1 | cat

[tusharmath]

Fixes #2621