OSC 52 clipboard read wired into IClipboardService

[mstrobel]

Adds system-clipboard read via the OSC 52 query/response round-trip and hooks it into the UI layer's IClipboardService (the write side shipped earlier).

Core

• DeviceResponseKind.Clipboard + the interpreter's OSC 52 response arm (payload <targets>;<base64>, consumer decodes — the PaletteColor precedent).
• ClipboardWriter.WriteQuery emits OSC 52 ; c ; ? ST.
• Family-gated ClipboardRead capability. Not probed at startup — probing would flash the per-read permission prompt supporting terminals gate the read behind; the cap means "worth attempting", and a denied/unsupported read simply never replies.

UI

• TerminalClipboardService.TryGetTextAsync: emits the query out-of-band, registers a one-shot device-response waiter, arms a frame-aligned UITimer for the timeout, subscribes BeginShutdown; the first of {reply, timeout, shutdown} completes and tears down all three (single-threaded on the UI loop). Decodes targets;base64 — empty selection → "", echoed ? → null.
• TextBox Ctrl+V / Shift+Insert kick the read (2 s timeout for the human-scale permission prompt) and insert via the FromPaste path; bracketed paste stays the primary inbound path. An in-flight guard coalesces a held/double chord (OSC 52 has no request id, so one reply completes every pending read); the completion routes faults through the app exception funnel.

Adversarial-review fixes (mutation-verified)

• Security — EmitDeviceResponse drops every device response while _inPaste. The classifier is a pure framing layer with no paste awareness, so an ESC embedded in pasted content reaches the OSC dispatcher mid-paste — a hidden ESC]52;c;… in copied web content would otherwise forge a Clipboard reply and satisfy a pending read, bypassing the terminal's permission gate (clipboard poisoning). The guard also closes the pre-existing OSC 4/10/11/12 color-forgery variant.
• Decode tolerates whitespace-wrapped / unpadded base64 (tmux, line-wrapping terminals) so a real reply isn't misread as "no clipboard"; a DEBUG breadcrumb separates decode-failure from no-reply.

Tests

Core interpreter + writer; ClipboardReadTests (round-trip, timeout, late reply, no-cap, empty, echoed query, UTF-8, malformed, whitespace/unpadded, multi-target, concurrent-reads fan-out, shutdown); TextBox C11_23a/b/c; the paste-forgery + outside-paste interpreter rows. Core 962 / UI 2948 / Bars 296 green.

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