feat(remap): add mouse controls actions

[blak0p]

Closes #84

Summary

• Adds Scroll Up/Down and DPI Cycle/DPI+/DPI− as capture-backed Mouse Controls remap actions.
• Allows those actions on Buttons 2–7 and visibly disables/rejects them for Button 1.
• Preserves the seven-slot report, explicit apply, ACK, persistence, reset, and DPI-marker contracts.

Changes

Area	Change
Protocol/X6/Desktop	Closed action IDs, validation, and ordered catalog.
Frontend	Mouse Controls category, labels, disabled Button 1 choices, and interaction coverage.
OpenSpec	Proposal, design, delta spec, tasks, and apply evidence.

Test Plan

• Focused protocol/desktop remap tests
• go test ./... — 323 tests
• go test -race ./internal/desktop/... — 81 tests
• go vet ./...
• Focused panel/hook tests — 14 tests
• (cd frontend && npm test) — 89 tests
• (cd frontend && npm run build)
• go build -o /tmp/attack-shark-mouse-controls ./cmd/x6configurator
• git diff --check

Review note

The maintainer accepted a 448-line single-PR exception because the cohesive change includes required tests and OpenSpec artifacts. Native reliability review approved this candidate.