Accept file drops on remote tmux mirror panes

[teamleaderleo]

Fixes #14896

Thanks @jeremywhelchel for the report and the precise trace through the drop path.

Root cause

Remote tmux mirror panes are rendered from the mirror's own nested BonsplitController, so a mirror pane's PaneDropContext carries a pane id the workspace tree has never seen. Since the pane-drop router landed, AppDelegate.paneDropContainer(for:) only accepted panes of the workspace or a Dock tree. It returned nil for mirror panes, and PaneDropTargetView refused the drag before any file-URL handling ran, so Finder and screenshot drops snapped back.

Fix

• RemoteTmuxWindowMirror conforms to PaneDropContainer. It resolves its own panes, so file drops reach the pane's hosted terminal (the same upload path cmux ssh panes and paste already use) and focus the dropped pane afterwards.
• The resolver falls back to the workspace's window mirror that owns the (panelId, paneId) pair.
• A new acceptsPaneTransfers container property (default true) lets the mirror decline surface, Vault, Cloud row and sidebar tool transfers at drag time, since tmux owns the mirror layout. Open-as-split file drops are declined for the same reason.

Tests

RemoteTmuxMirrorSplitRoutingTests.mirrorPaneDropContextResolvesToTheWindowMirror checks that a mirror pane context resolves to the mirror, reports a terminal text destination, rejects surface transfers, and does not resolve a mismatched panel/pane pair.

Not verified by hand against a live cmux ssh-tmux host; CI is the verifier.

🤖 Generated with Claude Code

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Summary by cubic

Fixes #14896: file drops on remote tmux mirror panes now reach the pane's hosted terminal instead of snapping back. The drop resolver previously returned nil for mirror panes—their contexts carry pane ids the workspace tree never sees—so PaneDropTargetView refused every Finder and screenshot drop before file-URL handling ran.

• RemoteTmuxWindowMirror conforms to PaneDropContainer and resolves its own panes; the resolver falls back to the workspace's mirror that owns the (panelId, paneId) pair, and the dropped pane is focused afterwards.
• New acceptsPaneTransfers (default true) lets the mirror decline surface, Vault, Cloud row, and sidebar tool transfers at drag time since tmux owns the mirror layout.
• Adds mirrorPaneDropContextResolvesToTheWindowMirror covering resolution, terminal text destination, surface rejection, and mismatched pane pairs.

^{Written for commit e5034bf. Summary will update on new commits.}

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Reviewing files that changed from the base of the PR and between 52dce98 and e5034bf.

📒 Files selected for processing (2)

• Sources/PaneDropContainer.swift
• cmuxTests/RemoteTmuxMirrorSplitRoutingTests.swift

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Merge receipt for e5034bfa9e: every check was green at merge (16 verified; 14 skipped by policy). Full suite runs on main after merge.