feat: FreqResponsePointwise — batch sweep with guaranteed single-point arithmetic

[jamestjat]

Why

FreqResponse switches long sweeps of delay-free state-space models to a transfer-function conversion (directFrequencySweepWorkLimit = 80) whose values are close but not bit-identical to the single-point path. Downstream consumers that need sweep results to reproduce single-point evaluations exactly have to mirror the internal work limit and chunk their sweeps — a coupling to an unexported constant that a dependency bump can silently break.

Concretely: internal/csadapter in APC.Exp.AspenAPC-Go carries directSweepWorkLimit = 80 plus a chunking loop, pinned by TestChunkedFreqResponseBitIdentical. This PR removes the need for both (monorepo task CM2QFO, found by DFDQAZ wave 48).

What

New exported method:

func (sys *System) FreqResponsePointwise(omega []float64) (*FreqResponseMatrix, error)

Contract: the value at each omega[k] is bit-identical to FreqResponse([]float64{omega[k]}), regardless of len(omega).

• Delay-free / IO-delay state-space path: per-ω direct solve (evalFrSSInto) with the delay phase applied per point — the exact single-point arithmetic.
• Per-point transfer-function fallback when the state-space solve fails at an isolated frequency (e.g. a pure integrator at ω=0), matching what a one-element FreqResponse call does for that point; the TF conversion is computed once and reused (deterministic).
• Internal-delay (LFT) and descriptor paths already evaluate one frequency at a time with batch-size-independent arithmetic and delegate to the existing evaluator unchanged.

Also adds a README API-table row.

Tests

frequency_pointwise_test.go pins the bit-identity guarantee (Float64bits comparison) across:

• sweep lengths 1 / 40 / 137 / 500 (below and above the work limit) for first-order, second-order + input delay, and static-gain systems
• discrete systems, MIMO with output delay, internal delay (LFT)
• the mixed per-point TF fallback (pure integrator with ω=0 in the grid)
• empty omega (nil result, matching FreqResponse)

Local validation: go fix ./... clean, go vet ./... clean, go test -count=1 -race ./... green.

Please review; do not merge/tag on my behalf — the monorepo adapter switch is staged and lands after a tagged release.

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