feat(tui): OSC 11 background-color query for theme auto-detect (theme iter 3)

[gerchowl]

Closes #176.

Summary

Iter 3 of the theme auto-detect chain. Resolver order is now:

flag > env > config > COLORFGBG > OSC 11 > dark default

Iter 2 (#151) shipped the COLORFGBG step and explicitly deferred OSC 11 because it needed raw-tty handling and a millisecond-grained timeout. This lands that piece on its own.

• New theme::osc11 submodule. Pure helpers (parse_osc11_reply, classify_rgb) plus a unix query_terminal_background driver that emits \x1b]11;?\x07, time-bounds the stdin read with libc::poll, parses the rgb:RRRR/GGGG/BBBB or rgb:RR/GG/BB reply with BEL or ST terminator, and classifies by CCIR-601 luminance against threshold 128.
• Theme module promoted to a directory: theme.rs -> theme/mod.rs.
• detect_terminal_background now chains COLORFGBG then OSC 11; each probe is isolated for testability.
• 150ms timeout const at module top with a breadcrumb to promote it to SCITADEL_OSC11_TIMEOUT_MS later if slow ssh links complain.

Safety / correctness

• Probe is gated behind IsTerminal on stdin AND stdout — no query fires under cargo test, cron, or redirected I/O.
• Termios is saved with tcgetattr before flipping to non-canonical / no-echo and unconditionally restored before return.
• libc::poll enforces the millisecond deadline; read is only called after POLLIN so the call is non-blocking by construction.
• Read loops on partial replies (tmux pass-through can fragment) but always honors the deadline.
• On any failure path (non-tty, timeout, malformed, stdio error) the function returns None and the resolver falls through to dark.

Dependencies

Zero new entries in Cargo.lock — libc is already transitive via crossterm/tokio/mio. Added as a direct cfg(unix)-gated dep on scitadel-tui so the module can call libc::poll / tcgetattr / tcsetattr directly.

Test plan

Automated (12 new tests; 50 total in scitadel-tui, full workspace green):

• parse_osc11_reply x6: 4-byte BEL light bg, 2-byte BEL dark bg, 4-byte ST terminator, malformed (5 sub-cases), empty buffer, leading-noise tolerance.
• classify_rgb x7: black, white, warm-cream Dalton-bright bg, Solarized-dark bg, pure red, pure green, threshold edge case.
• osc11_skipped_when_stdin_not_a_tty: resolve(None, "auto") returns under 50ms (well below the 150ms timeout) when stdin is a pipe — fences against the resolver ever hanging the test runner. Reuses the existing ENV_LOCK (fix(tui): serialize env-mutating theme tests (#165) #169 carry-over) so it serializes with the COLORFGBG / SCITADEL_THEME mutating tests.
• cargo fmt --all -- --check
• cargo clippy --workspace --tests -- -D warnings
• cargo test --workspace

Manual (requires a real OSC-11-supporting tty; not fakeable in CI):

• SCITADEL_THEME= COLORFGBG= scitadel tui on foot/kitty/alacritty/modern xterm — light terminal toast reports dalton-bright (auto), dark terminal reports dalton-dark (auto).
• Same on a terminal that ignores OSC 11 (older tmux without pass-through) — startup delay <= 150ms then dalton-dark (auto).

[tape-exempt: detection runs at startup before TUI render; observable behavior verified by parser/luminance unit tests; full e2e requires a real OSC-11-supporting terminal]