[Draft] feat(vardiff): decline-safe vardiff champion + simulation framework + proof

.github/workflows/vardiff-sim.yaml

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+name: vardiff_sim
+
+on:
+  pull_request:
+    branches:
+      - main
+    paths:
+      - 'sv2/channels-sv2/sim/**'
+      - 'sv2/channels-sv2/src/vardiff/**'
+      - 'sv2/channels-sv2/src/target.rs'
+      - '.github/workflows/vardiff-sim.yaml'
+
+jobs:
+  # Fast tests in the sim crate. Always run.
+  test-fast:
+    name: cargo test --lib
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: sv2/channels-sv2/sim
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install stable Rust
+        uses: dtolnay/rust-toolchain@stable
+        id: toolchain
+
+      # The repo root's `rust-toolchain.toml` pins 1.75, but the sim
+      # crate's Cargo.lock is v4 format (needs newer rustup). Override
+      # explicitly so the sim crate gets the stable toolchain.
+      - name: Override toolchain
+        run: rustup override set ${{ steps.toolchain.outputs.name }}
+        working-directory: sv2/channels-sv2/sim
+
+      - name: Cache cargo registry
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            sv2/channels-sv2/sim/target
+          key: ${{ runner.os }}-vardiff-sim-${{ hashFiles('sv2/channels-sv2/sim/Cargo.lock') }}
+
+      - name: Build sim crate
+        run: cargo build --release --verbose
+
+      - name: Fast unit tests
+        run: cargo test --lib --verbose
+
+  # Slow regression check: `cargo test --release --lib -- --ignored` runs the
+  # three drift guards (~35s total), each with CI-aware tolerance budgets:
+  #   - classic_algorithm_no_regression  — the classic comparison anchor
+  #     (baseline_classic.toml).
+  #   - champion_algorithm_no_regression  — the SHIPPED champion's typical-
+  #     behavior grid (baseline_champion.toml).
+  #   - champion_clears_decline_gate      — the champion's decline-safety MARGIN
+  #     (its selection criterion; not covered by the grid baselines).
+  test-regression:
+    name: regression vs checked-in baseline
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: sv2/channels-sv2/sim
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install stable Rust
+        uses: dtolnay/rust-toolchain@stable
+        id: toolchain
+
+      # The repo root's `rust-toolchain.toml` pins 1.75, but the sim
+      # crate's Cargo.lock is v4 format (needs newer rustup). Override
+      # explicitly so the sim crate gets the stable toolchain.
+      - name: Override toolchain
+        run: rustup override set ${{ steps.toolchain.outputs.name }}
+        working-directory: sv2/channels-sv2/sim
+
+      - name: Cache cargo registry
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            sv2/channels-sv2/sim/target
+          key: ${{ runner.os }}-vardiff-sim-regression-${{ hashFiles('sv2/channels-sv2/sim/Cargo.lock') }}
+
+      - name: Slow regression test
+        run: cargo test --release --lib -- --ignored
+
+  # Verify that the binaries still build (compare-algorithms,
+  # generate-baseline, trace-trial, sweep-ewma-tau). Doesn't run them
+  # (full sweep takes ~150s).
+  build-binaries:
+    name: cargo build --release (all bins)
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: sv2/channels-sv2/sim
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install stable Rust
+        uses: dtolnay/rust-toolchain@stable
+        id: toolchain
+
+      # The repo root's `rust-toolchain.toml` pins 1.75, but the sim
+      # crate's Cargo.lock is v4 format (needs newer rustup). Override
+      # explicitly so the sim crate gets the stable toolchain.
+      - name: Override toolchain
+        run: rustup override set ${{ steps.toolchain.outputs.name }}
+        working-directory: sv2/channels-sv2/sim
+
+      - name: Cache cargo registry
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+            sv2/channels-sv2/sim/target
+          key: ${{ runner.os }}-vardiff-sim-bins-${{ hashFiles('sv2/channels-sv2/sim/Cargo.lock') }}
+
+      - name: Build all binaries
+        run: cargo build --release --bins --verbose

sv2/channels-sv2/sim/.gitignore

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+# Build artifacts (sim is its own Cargo workspace; root .gitignore
+# covers the parent workspace's target/, this entry covers ours).
+/target
+
+# macOS finder noise
+.DS_Store
+
+# Editor swap files
+*.swp
+*.swo
+*~
+
+# Locally generated reports & traces — regenerate via `cargo run`, no
+# need to track. The checked-in baselines are explicitly named.
+/trace-*.txt
+/scratch-*
+# Sweep/comparison reports and charts (regenerate via the sweep / radar
+# / trajectory bins; the analysis lives in commit messages / docs, not
+# the raw output files).
+/*_sweep.md
+/*_sweep_big.md
+/confirm_*.md
+/champion_weights.md
+/eta_sweep.md
+/eta_z_joint_sweep.md
+/iterative_eval.md
+/pareto.md
+/*.svg
+/docs/*.svg
+/compare_out*/
+/sweep_out/
+
+# Cross-algorithm comparison baselines are regenerated on demand by
+# `compare-algorithms` (that binary is their archive). Only the two CI
+# regression fixtures are tracked; ignore every other baseline_* so a
+# comparison run does not re-stage the bulky frozen snapshots that used
+# to be force-added here.
+/baseline_*.toml
+/baseline_*.md
+!/baseline_classic.toml
+!/baseline_classic.md
+!/baseline_champion.toml
+!/baseline_champion.md
+
+# Optional: cargo-semver-checks scratch directory
+.semver-checks/