Address high-confidence clang analyzer findings from warning report (#10355)

The clang-tidy warning report surfaced a small set of high-confidence
analyzer findings with straightforward fixes: intentional null
dereference in debug-only crash paths, undefined oversized shifts in
`mpff`, uninitialized state in sorting-network setup, and `% 0` in a
unit test edge case.

- **Debug crash path**
- Replaced intentional null writes in `src/util/debug.cpp` with an
explicit crash helper based on `SIGSEGV`/`abort`.
- Keeps the failure mode intentional without relying on undefined
pointer dereference.

  ```c++
  [[noreturn]] static void force_segfault() {
      std::raise(SIGSEGV);
      std::abort();
  }
  ```

- **`mpff` integer extraction**
- Hardened `get_uint64` / `get_int64` against analyzer-reported
oversized right shifts.
- Computes the shift count in `int64_t`, asserts the valid range, and
guards the shift site.

- **Sorting-network initialization**
- Initialized `psort_nw::m_t` in the constructor to avoid
uninitialized-object diagnostics on construction paths that inspect
state before later assignment.

- **Test-only edge cases**
- Added an early return in `src/test/total_order.cpp` for `sz == 0` to
avoid `% 0` in randomized loops.
- Removed the extra trailing semicolon pattern around the `find_q`
namespace in `src/test/var_subst.cpp`.

- **Scope**
- Focused only on localized, semantics-preserving fixes from the warning
artifact.
- Left broader architectural warnings (for example,
constructor/destructor virtual-call diagnostics) out of this change.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

src/util/sorting_network.h

@@ -224,7 +224,7 @@ Notes:
             }
         };
 
-        psort_nw(psort_expr& c): ctx(c) {}
+        psort_nw(psort_expr& c): ctx(c), m_t(GE) {}
 
         sorting_network_config& cfg() { return m_cfg; }
 
@@ -1497,4 +1497,3 @@ Notes:
             }
         }
     };
-