Generate contract types, pin fixtures to spec, add build and ADRs

Completes FLUID-WP-0002. The record types in internal/contract are
generated from schemas/ by a dependency-free generator; fixtures
transcribed from the spec's worked examples validate against those
schemas and round-trip through the generated types with
DisallowUnknownFields, so a spec change that misses the schemas fails
CI rather than drifting silently.

Adds identifier prefix helpers, Makefile, GitHub Actions, and five ADRs
recording the Go choice, out-of-process attachment, the wire contract as
boundary, the evidence store, and revision identity.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014KmVxhJ35tCo7rE7UnLwWu

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 1116572@bnt-lap001
Assistant-Session: 8ba9bb93-a72a-4883-b189-2499cce5c400

internal/contract/ids.go

@@ -0,0 +1,91 @@
+package contract
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Identifier prefixes from FluidHypothesisRevisionSchema.md section 16.
+//
+// Implementations may use UUIDs internally; these human-readable prefixes exist
+// so that operational tooling and audit trails stay legible to people.
+const (
+	PrefixHypothesis         = "H-"
+	PrefixRevision           = "R-"
+	PrefixExperiment         = "E-"
+	PrefixPressure           = "P-"
+	PrefixBackendRequirement = "BR-"
+	PrefixDecision           = "D-"
+	PrefixEvent              = "EV-"
+	PrefixFeedback           = "F-"
+	PrefixCohort             = "C-"
+)
+
+// EntityKind names the FLUID artifact an identifier refers to.
+type EntityKind string
+
+const (
+	KindHypothesis         EntityKind = "hypothesis"
+	KindRevision           EntityKind = "revision"
+	KindExperiment         EntityKind = "experiment"
+	KindPressure           EntityKind = "pressure"
+	KindBackendRequirement EntityKind = "backend_requirement"
+	KindDecision           EntityKind = "decision"
+	KindEvent              EntityKind = "event"
+	KindFeedback           EntityKind = "feedback"
+	KindCohort             EntityKind = "cohort"
+)
+
+// prefixOrder matters: "BR-" must be tested before "B"-less single letters
+// would otherwise mis-claim it, and "EV-" before "E-".
+var prefixOrder = []struct {
+	prefix string
+	kind   EntityKind
+}{
+	{PrefixBackendRequirement, KindBackendRequirement},
+	{PrefixEvent, KindEvent},
+	{PrefixHypothesis, KindHypothesis},
+	{PrefixRevision, KindRevision},
+	{PrefixExperiment, KindExperiment},
+	{PrefixPressure, KindPressure},
+	{PrefixDecision, KindDecision},
+	{PrefixFeedback, KindFeedback},
+	{PrefixCohort, KindCohort},
+}
+
+// KindOf reports which FLUID artifact an identifier names.
+//
+// Audit trails carry bare identifiers across record boundaries, so being able to
+// classify one without knowing where it came from keeps trace reconstruction
+// from needing a lookup table at every hop.
+func KindOf(id string) (EntityKind, bool) {
+	for _, p := range prefixOrder {
+		if strings.HasPrefix(id, p.prefix) {
+			return p.kind, true
+		}
+	}
+	return "", false
+}
+
+// ErrWrongKind reports an identifier used in the wrong position.
+type ErrWrongKind struct {
+	ID   string
+	Want EntityKind
+	Got  EntityKind
+}
+
+func (e *ErrWrongKind) Error() string {
+	if e.Got == "" {
+		return fmt.Sprintf("identifier %q has no recognized FLUID prefix, wanted a %s id", e.ID, e.Want)
+	}
+	return fmt.Sprintf("identifier %q is a %s id, wanted a %s id", e.ID, e.Got, e.Want)
+}
+
+// RequireKind checks that id names the expected artifact.
+func RequireKind(id string, want EntityKind) error {
+	got, ok := KindOf(id)
+	if !ok || got != want {
+		return &ErrWrongKind{ID: id, Want: want, Got: got}
+	}
+	return nil
+}