20.4: Mask stack UI — the CS6 mask panel #76

Objective. Expose the mask stack in the FireCraft shell as a CS6-shaped panel: one row per mask level, in stack order, with per-level enable, delete, combine mode, and selection coupling to the canvas.

Why. FireCraft's engine already carries mask commands (crates/engine/src/catalog/mask.rs) and mask plumbing in the document model (crates/doc/src/fwmap/reader.rs), but a user cannot see or manipulate a mask at all. A mask that exists only in the file format is not a feature: it is data the user cannot inspect, disable, reorder, or reason about. The shell is where FireCraft stops being a file-format library and becomes a daily-driver tool.

Current state. The stack model, its FWMap persistence, the compositor fold and the level panel are implemented on branch feat/mask-stack-ui-v2 and in flight as PR #113 (head 44b383a, merged with main and conflict-free). The merge is gated on the two CI receipts; it is not on main yet. The engine catalog exports element_mask_stack(document, id) -> Vec<MaskLevel>, and the shell panel is wired to the registry command surface.

Authority. docs/tasks/T9-mask-stack.md; decisions 0092 (wire contract) and 0093 (level model); the CS6 reference install's Properties panel mask section as the behavioural authority; docs/planning/generated/dom-parity.md for the command-bound count.

Work.

1. The mask panel lists the document's masks in stack order, matching the CS6 mask panel's section.
2. Per-mask controls: enable/disable, delete, and the combine mode (add / subtract / intersect / invert) with a preview of the mask shape.
3. Selecting a mask in the panel selects the corresponding object on the canvas.
4. Everything is exposed as registry commands, reachable from the menu tree and the command palette — no panel-local behaviour.

Acceptance.

• A document with two masks shows two rows; toggling a mask's enable flag changes the rendered output.
• The combine mode of a selected mask is editable and takes effect.
• No inert menu rows are added (the inert_count_never_rises test caps them at 8).
• The reachability floor does not regress (docs/planning/generated/dom-parity.md regenerated with no new unreachable share).

Verification. cargo run -p xtask -- ci green on the exact PR head; the mask stack's unit tests cover fold/unfold and FWMap round-trip; the workspace clippy and test suite pass.

Boundaries. The mask stack is a FireCraft extension — CS6 has one element mask per object and no stack at all. The stack's shape follows the modern raster editors (Photoshop layer masks, Affinity mask stacks): per-level operators over the coverage below. Do not present it as a CS6 parity feature.

Tracking. Task #143; PR #113.

Next action (owner: shell). Land PR #113 on the receipt of both CI gates, then take the follow-ups in #114 — the panel's screenshot acceptance against the CS6 Properties panel, and the compositor's mask sampling resolution.