fix(perturbation-sim): degenerate-grid + key-cardinality guards (review #511)

Addresses the open review findings on the merged #511:

- examples/calibrate.rs: guard degenerate grids up front. `k = 24.min(m)` is 0
  when m==0, so `m / k` panics (divide by zero); `eigenvector(1)` / `m - 1` also
  break on n<2. Exit cleanly with a message instead. (CodeRabbit Major)
- src/hhtl.rs: basin_lambda2 asserts keys.len() == grid.n. A mismatched slice
  silently produced wrong basin groupings + lambda2; fail fast at the API
  boundary. (CodeRabbit Major)
- .claude/board/TECH_DEBT.md: reflow the wrapped `#507` so it is not parsed as
  an ATX heading (markdownlint MD018). (CodeRabbit Minor)

Already-fixed before merge (no change needed): Codex P2 — `infight` is 4-bit
(columns.rs `spec("infight", 4, ...)`, the certified width per the §10 note).
Deferred: the TECH_DEBT.md append-only addendum-placement nit (CodeRabbit Major)
is a board-hygiene reorganization, left to a focused governance pass rather than
churned inside a code-fix PR.

Gates: fmt + clippy -D warnings + test green (75 lib tests) on the standalone
perturbation-sim crate.

https://claude.ai/code/session_016b33swuXE23hKtqxsHu9p1

Author: claude

.claude/board/TECH_DEBT.md

@@ -175,9 +175,9 @@ flags. Separately, #507 left `intervene_counterfactual.rs:133/165` calling the
 `8131c480` lives on the unmerged `claude/continue-ndarray-x0Oaw`) — that WARNS, does
 not fail (v1 default routes through the canonical mapping per I-LEGACY-API-FEATURE-
 GATED); tracked here as a separate latent item, not fixed on this CI branch.
-Cross-ref: `.github/workflows/rust-test.yml` (now both jobs at `debuginfo=0`); PR
-#507 (`0c6ef02c`); `claude/continue-ndarray-x0Oaw` (the pending ce64-v2 consumer
-migration).
+Cross-ref: `.github/workflows/rust-test.yml` (now both jobs at `debuginfo=0`);
+PR #507 (`0c6ef02c`); `claude/continue-ndarray-x0Oaw` (the pending ce64-v2
+consumer migration).
 
 ### TD-UNBUNDLE-FROM-1 — `unbundle_from` is NOT the inverse of `bundle_into` (2026-06-07)
 

crates/perturbation-sim/examples/calibrate.rs

@@ -123,11 +123,19 @@ fn main() {
     };
     let n = grid.n;
     let alive = vec![true; grid.edges.len()];
+    let m = grid.edges.len();
+    // Degenerate grids would divide by zero (`k = 24.min(m) = 0` → `m / k`) and
+    // break the eigenvector / `m - 1` assumptions below. Guard up front.
+    if n < 2 || m == 0 {
+        eprintln!(
+            "calibrate requires a connected grid with at least 2 buses and 1 line (got n={n}, m={m})"
+        );
+        std::process::exit(2);
+    }
 
     // Ground truth = the study's 5-factor contingency matrix on the real core.
     let base = symmetric_eigen(&grid.laplacian_of(&alive), n);
     let v2 = base.eigenvector(1);
-    let m = grid.edges.len();
     let mut sens: Vec<(usize, f64)> = (0..m)
         .map(|e| {
             let d = v2[grid.edges[e].from] - v2[grid.edges[e].to];

crates/perturbation-sim/src/hhtl.rs

@@ -106,6 +106,13 @@ pub fn hhtl_keys(grid: &Grid) -> Vec<HhtlKey> {
 /// Per-leaf-basin algebraic connectivity `λ₂` keyed by HHTL address — the topology
 /// "value" the key indexes (read once from the spectrum, deterministic).
 pub fn basin_lambda2(grid: &Grid, keys: &[HhtlKey]) -> HashMap<HhtlKey, f64> {
+    assert_eq!(
+        keys.len(),
+        grid.n,
+        "basin_lambda2 requires exactly one HHTL key per grid node (got {} keys for {} nodes)",
+        keys.len(),
+        grid.n
+    );
     let mut groups: HashMap<HhtlKey, Vec<usize>> = HashMap::new();
     for (n, k) in keys.iter().enumerate() {
         groups.entry(*k).or_default().push(n);