OSC 11 terminal background-color detection + DECRQM 2026 probe.

Emits an OSC 11 query (ESC ] 11 ; ? BEL) and a DECRQM query for mode 2026 (CSI ? 2026 $ p, synchronized output), followed by a primary Device Attributes probe (CSI c) as the sentinel. Every terminal answers DA, so a DA reply that arrives without a wanted reply means the terminal does not support that query and callers fall back conservative (silence is the failure mode, F0 §2).

Replies arrive asynchronously on the input stream, interleaved with keystrokes. scan/1 routes the chunk through Raxol.Terminal.Capabilities.ReplyScanner (grammar dispatch, both OSC terminators, leak-free residual) so reply bytes never leak into key-event parsing. A parsed DECRQM 2026 reply is noted on Raxol.Terminal.Capabilities -- Capabilities.sync_output?/0 is the one public emit-gate render paths consult. scan/1 keeps its original {result, cleaned} contract for the driver.

detected_background/0 is now a delegating shim over the unified Raxol.Terminal.Capabilities session record (native-palette-riding): it reads Capabilities.background/0 first, falling back to this module's own :persistent_term cache (written by store/1) only when no Capabilities record has been cached yet. store/1 is unchanged and still the entry point older callers use directly. The parse functions (parse_color/1 and friends) stay live -- Raxol.Terminal.Capabilities's own scanner/classifier path reuses them. Retirement planned: once every caller reads Capabilities.background/0 (or ground/0) directly, this module's :persistent_term fallback and store/1 go away and only the parse helpers, if still needed, get a new home.

Higher layers (e.g. salience theming) convert the background to a ground lightness -- see Raxol.UI.Theming.SalienceTheme.detect_ground/0.