Clipboard paste must preserve unrelated terminal primary-selection affinity

[c-wri]

Observed

During live COSMIC Terminal qualification, grabbing selected text immediately after a successful ClipReg paste could fail even though the service remained command-ready.

The runtime path explains the behavior:

• COSMIC Terminal profile pastes with CTRL+SHIFT+V, i.e. the regular clipboard.
• command_paste() nevertheless stages both SEL_CLIPBOARD and SEL_PRIMARY whenever global stage_primary_for_paste is enabled.
• Paste restore then unconditionally sets primary_affinity_valid = false.
• The next terminal primary-first grab cannot use the affine primary-selection fast path and falls into application Copy fallback instead.

The live journal shows successful pastes entering paste-stage-primary, followed by subsequent grabs taking the fallback transaction path rather than immediate primary capture.

Violated invariant

A paste transaction should mutate only the selection domains required by the selected paste behavior. A clipboard-only paste accelerator must not disturb an unrelated primary selection or discard trustworthy primary affinity merely as collateral staging state.

Investigation / design direction

Make the selection source consumed by a paste accelerator explicit enough that transaction staging/restoration follows that contract rather than globally staging both selections. Avoid brittle assumptions for configurable application profiles; if selection semantics need to become profile data, design that contract directly rather than inferring arbitrary custom chords by string coincidence.

The synthetic terminal-input leakage from generic Copy probing is tracked separately in #5.

Acceptance

• COSMIC Terminal CTRL+SHIFT+V paste stages/restores the regular clipboard without replacing primary selection solely for paste compatibility.
• A trustworthy terminal primary selection/affinity present before a clipboard-only paste remains usable for an immediate primary-first grab afterward, unless the application itself changed it.
• Primary-selection paste paths (for example an explicitly supported Shift+Insert policy) continue to stage the primary selection when actually required.
• Add a durable policy/lifecycle regression proving clipboard-only paste does not invalidate unrelated primary affinity.
• Live COSMIC Terminal qualification: select text, paste a register, then grab the still-valid/newly selected terminal text without requiring a synthetic generic Copy probe or service restart.

This blocks v0.2.0-rc.1 because supported terminal grab/paste composition is part of live qualification.