Nothing can be copied out of the console: the log is not selectable and the clipboard write path does not exist

[wallstop]

DISCLOSURE: LLM-GENERATED TEXT

Problem

Nothing can be copied out of the console. The log view is the only
place a message, a command echo, an argument error, or a variable
value ever appears, and none of it can be selected.

The package's only clipboard integration is the paste direction
(TextFieldPaste.cs reads GUIUtility.systemCopyBuffer). A
GUIUtility.systemCopyBuffer = ... write appears nowhere in Runtime/.

Evidence

• Runtime/CommandTerminal/UI/TerminalUI.cs:2689 - every log line is
  built as a Label. A Label is not selectable, and no
  pointer-selection handler is registered anywhere in Runtime/.
  Styles/BaseStyles.uss styles it as plain text.
• Runtime/CommandTerminal/UI/TerminalUI.cs:2713 and :2727 - the
  case TextField and case Button arms of the log refresh are dead
  branches; no code path creates either any more.
• Runtime/CommandTerminal/Backend/BuiltinCommands.cs - the built-in
  set has clear-console and clear-history but nothing that reads the
  log out.
• The command line has the same gap in the other direction: you can
  select half a line with shift+arrows and then not copy it, not cut
  it, and not undo the keystroke that replaced it. Nothing in the
  package implements an undo stack; the field is the engine's.

Why it matters

The paste feature that landed in #180 was justified by exactly this
case read backwards ("a developer copying a save name, a stack trace,
or a chat line has to type it"). The other direction is the same need:
you see Command 'give' threw ArgumentException: ... in a device build
and your options are to retype it or screenshot the game window.

What a fix has to choose

1. Selection in the log. The honest fix: TextSelectable labels
   plus a pointer/keyboard selection model, and a Ctrl+C path. This is
   real work and it is the only option that gives selection semantics.
2. Commands, not selection. copy-last and copy-log [n], which
   put the newest line (or the last n) on the system clipboard. Much
   smaller, covers the common case of "copy that one error", and does
   not need the log to be selectable.
3. Both, with the commands first.

On WebGL the clipboard is unavailable, exactly as it is for paste
(#180): the command should answer that it could not, not silently do
nothing.

Acceptance

A developer in Play Mode can put a log line's text on the system
clipboard. On tvOS, or on a platform that will not answer, the attempt
reports that rather than failing silently. No selection regression: the
log still renders and still follows its tail (#194).

[wallstop]

DISCLOSURE: LLM-GENERATED TEXT

Landed in #201

copy-last and copy-log [n] put the newest line, or the last N, on the
system clipboard, and each answers in the console like every other command. A
clipboard write is a request a platform can decline, so the write is read back
and compared, and a copy that did not happen is reported rather than silently
doing nothing. TerminalClipboard is now the one place that names
GUIUtility.systemCopyBuffer, and the paste path reads through it.

Two limits are stated rather than papered over:

• A read-back is not an acknowledgement. A browser clipboard is promise-based,
  so a write that did succeed can read back the old value, and copying the
  same window twice reports success whether or not the second write landed.
  This is the only portable signal there is.
• The clipboard is a main-thread engine API. CommandShell.RunCommand is
  public and can be driven off the main thread; the copy commands answer on
  whatever thread ran them, which is the shell's existing contract.

Not landed

The log is still not selectable. Option 1 in this issue - TextSelectable
labels, a pointer and keyboard selection model, and Ctrl+C over the log - is
real work and is the only option that gives selection semantics. What landed
covers "copy that one error" and "send me the log", not "copy these four
characters of it". The command line also still has no copy of a selection, no
cut, and no undo.

PLAN.md keeps that as the first open user-facing item.