feral v0.7.0

v0.7.0 — opt-in instrumentation accessors

Minor release for #52.
New public surface on Solver for downstream IPM debugging of
linear-solver behavior, without re-running with cargo flamegraph
or instrumenting forks.

Added

Phase A — always-on snapshot. Solver::last_factor_stats()
returns Option<FactorStats> populated after every successful
factor(). Fields: nnz_a, nnz_l, fill_ratio, inertia,
min_abs_pivot, max_abs_pivot, pattern_reused, scaling_info.
No gating flag — the two extra integer writes per factor() are
cheaper than a gate check would be.

Phase B — opt-in profiler. Solver::with_profiling(true)
enables per-supernode numeric timings and one-shot symbolic timings,
read back via Solver::profile_report() and
Solver::symbolic_profile_report(). The symbolic report is
populated only on cache-miss factors — None on cache hits is the
unambiguous "did symbolic actually run" signal.

Performance

Default is false; when off, no profiler arcs are allocated and the
code path is byte-identical to a pre-issue-52 build. Measured by
benches/issue52_overhead.rs on tridiagonal SPDs
(n ∈ {64, 256, 1024}, sequential, 30 samples × 3 s, dev machine):

n	main baseline	default-off	profiling-on
64	260.6 µs	257.6 µs	258.9 µs
256	348.2 µs	345.2 µs	347.0 µs
1024	719.6 µs	714.7 µs	709.1 µs

Default-off vs main baseline: within ±1.2% (often faster — noise).
Profiling-on vs default-off: within ±1%. Both deltas sit inside the
criterion noise band on tridiagonal workloads.

Deferred

• Thread-local accumulator for parallel-driver profiler contention
  (plan §B2b) — documented as escape hatch; revisit only if a real
  IPM workload regresses with profiling on.
• Phase C residual-norm / refinement-step accessors — out of scope
  for this issue.