feat: Add digital, dry, ink, and paint brush presets; implement regression testing hooks

- Introduced new crates for digital brushes, dry media brushes, ink brushes, and paint brushes, each containing various presets.
- Updated the menus to include "Paste Special" and "Paste as New Layer" options.
- Enhanced the feature scorecard tests to validate selection texture encoding.
- Added a regression gate workflow for automated testing on push and pull requests.
- Implemented Git hooks for pre-commit checks to ensure code formatting and run tests before commits.
- Created a settings file for local configurations.

scripts/precommit-test-gate.sh

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+set -Eeuo pipefail
+
+REPO_ROOT="$(git rev-parse --show-toplevel)"
+cd "$REPO_ROOT"
+
+mapfile -d '' staged_files < <(
+    git diff --cached --name-only --diff-filter=ACMRDTUXB -z
+)
+
+if (( ${#staged_files[@]} == 0 )); then
+    echo "No staged files; commit regression tests are not required."
+    exit 0
+fi
+
+staged_rust_sources=()
+for path in "${staged_files[@]}"; do
+    printf '  staged input: %s\n' "$path"
+    if [[ "$path" == *.rs && -f "$path" ]]; then
+        staged_rust_sources+=("$path")
+    fi
+done
+
+dirty_inputs=()
+while IFS= read -r -d '' path; do
+    dirty_inputs+=("$path")
+done < <(git diff --name-only --diff-filter=ACMRDTUXB -z)
+while IFS= read -r -d '' path; do
+    dirty_inputs+=("$path")
+done < <(git ls-files --others --exclude-standard -z)
+
+if (( ${#dirty_inputs[@]} > 0 )); then
+    echo "Commit blocked: unstaged or untracked files would make staged test results ambiguous:" >&2
+    printf '  %s\n' "${dirty_inputs[@]}" >&2
+    echo "Stage or remove those files, then retry the commit." >&2
+    exit 1
+fi
+
+command -v cargo >/dev/null 2>&1 || {
+    echo "Commit blocked: cargo is unavailable." >&2
+    exit 1
+}
+
+if (( ${#staged_rust_sources[@]} > 0 )); then
+    echo "Checking staged Rust source formatting..."
+    rustfmt --edition 2021 --check "${staged_rust_sources[@]}"
+fi
+
+echo "Running deterministic workspace tests..."
+cargo check --locked --workspace --exclude hcie-io --examples
+cargo test --locked --workspace --exclude hcie-io --lib --tests -- \
+    --skip benchmark_4k_stroke_on_multilayer_document
+
+echo "Mandatory Rust regression gate passed."