feat(harness): Shift+click extends selection in composer + queue editor (plan #752)

docs/harness-limits.md

@@ -62,6 +62,7 @@ closed).
 | **Ctrl+**/**Cmd+/** | Toggle the in-canvas **help overlay** (TEAL panel over the transcript band) |
 | **Ctrl+Shift+Space** | Arm the **leader** prefix (800 ms). Within the window press **`?`** to toggle help; **`t`** toggles **thinking default-collapsed** (collapse thinking by default, including the active Busy turn; in-memory only, resets to on on reload/New/Clear); **Escape** cancels; an unmatched key swallows (never lands in the prompt); a reserved browser chord still yields to the browser |
 | Tab / Ctrl+Left / Ctrl+Right | DOM nav / text caret (not harness chords; Ctrl+Left/Right are word-jumps, left to the textEntry) |
+| **Shift+click** (composer / queue-row editor focused; desktop only) | **Range-selects** the text in that `textEntry` field — a stock web behavior the canvas was missing. Click to place the caret (or drag-select a base range), then **Shift+click** to extend the selection from an anchor edge to the clicked point; further Shift+clicks keep extending from the same anchor edge, and a Shift+click never counts toward the next word/line double-click. The range uses the **existing TEAL `text_select` fill** (no new hex). **Ctrl/Cmd+C** then copies that substring to the system clipboard (the chord stays browser-reserved). A **plain** (non-shift) click still moves the caret and clears the range. Touch/mobile has no shift key — **📋** stays the reliable copy path and canvas Shift+click is documented desktop-only |
 | Composer focus | Requested on ready and after each send |
 
 ## Touch / mobile (~390px)
@@ -201,6 +202,7 @@ see Group boundaries).
 | Clipboard payload | **Message source** UTF-8 as stored in the Wasm ring (markdown as produced) — not re-serialized paint colors or “… N more” chrome |
 | Drag-select | Best-effort **within a single** `textLayout` only (rich MD is many widgets per body). Whole-reply drag-select is not the product path |
 | Cross-message selection | Not supported |
+| Composer / queue-row editor Shift+click | Desktop **Shift+click** range-selects within the focused **editable** `textEntry` field (composer or queue-row editor), and **Ctrl/Cmd+C** copies that substring. In-canvas **read-only** `textLayout` (rich MD is many widgets per body) is still single-`textLayout`-bounded drag-select only — Shift+click there is a tracked child follow-up, not this surface |
 | Composer paste | Supported via dvui text entry when the composer is focused. Composer is **multi-line** — pasted newlines are preserved; submission is normalized (CRLF → LF) and clamped to the prompt submit cap (`SUBMIT_CAP`). Leading whitespace / blank first lines in a paste are **preserved** on submit (not stripped); only a wholly blank/whitespace prompt is rejected |
 | Mobile / touch | **📋** (copy) is the supported path; multi-block drag-select on canvas is unreliable |
 | Secure context | System clipboard write needs https (or localhost); silent no-op possible if the browser blocks clipboard |