API reference — quiver/dispatch.h (Surface C)

ISA query/override and version introspection (API-DISP-001..005). Contracts mirror PRD 04 §4; lifecycle semantics in the dispatch architecture page. Introduced: v0.1. All functions are noexcept, allocation-free, and thread-safe.

API	Signature	Contract
API-DISP-001	Isa active_isa() noexcept	The tier dispatch would select now: highest CPU-supported, capped by QUIVER_ISA and any active override.
API-DISP-002	bool cpu_supports(Isa) noexcept	CPU and OS can execute the tier (XGETBV state checks on x86). kScalar is always true; ARM never reports kAvx*, x86 never reports kNeon. Out-of-range enum values return false.
API-DISP-003	bool set_isa_override(Isa) noexcept / void clear_isa_override() noexcept	Caps dispatch at the tier; false + no state change if unsupported (kScalar always accepted). Takes effect via the policy epoch; calls concurrent with a change may run under either policy — both correct (REQ-DISP-009). For benchmarking/diagnostics.
API-DISP-004	void warmup() noexcept	Eagerly resolves every dispatch entry under the current policy (and forces feature detection + the once-only QUIVER_ISA read). Idempotent.
API-DISP-005	Version version() noexcept / const char* version_string() noexcept	Compile-embedded version; the string has static storage duration. Matches the CMake project version by construction (both derive from config.h).

Example (from the test suite)

TEST(Dispatch, OverrideRoundTrip) {
  const Isa hw = quiver::active_isa();
  ASSERT_TRUE(quiver::set_isa_override(Isa::kScalar));  // kScalar always accepted
  EXPECT_EQ(quiver::active_isa(), Isa::kScalar);
  quiver::clear_isa_override();
  EXPECT_EQ(quiver::active_isa(), hw);  // hardware+env policy restored
}

Environment variable

QUIVER_ISA ∈ {scalar, neon, avx2, avx512} (case-sensitive), read exactly once at first policy computation; unrecognized or unsupported values are ignored (REQ-DISP-005).

Validation: tests/unit/test_dispatch.cpp (env matrix via per-process probes, override round-trip, monotonicity, TSan concurrency). Benchmarks: bench_dispatch measures the dispatch-overhead contract (REQ-DISP-003) from M3.