feat: meter kind-as-option + platform-infra price override

[I-am-nothing]

Milestone D SDK follow-up (PR #1b), stacked logically on the merged declaration-first metering work (#1 / v0.2.2). Two changes to ms.Meter, plus the validation that makes them safe.

What changed

1. Kind is now an OPTION, not a positional argument.
ms.Meter(name string, opts ...ms.MetricOption). ms.Counter / ms.Gauge become ms.MetricOptions that set the kind, so the call site reads exactly the same:

ms.Meter("orders.placed", ms.Counter, ms.Unit("order"), ms.Price(50_000))

ms.Unit / ms.Price are unchanged.

2. Platform-infra customer-price override.
A reserved infra.* / platform.* name — previously rejected at declaration — may now be declared with ms.Price only, to override what the module's customer is billed for that platform-measured infra:

ms.Meter("infra.compute.ms", ms.Price(0)) // absorb platform compute into your own pricing

This is a pure Plane-2 (customer-facing) choice. Kind/unit on these names are platform-owned, and the developer still owes the platform the measured COGS regardless.

Validation (panic at declaration, like ValidateName)

• a custom (non-reserved) name MUST pass exactly one kind option → panic if missing, or if both ms.Counter and ms.Gauge are given;
• a reserved name may carry ms.Price ONLY → panic if a kind or unit option is passed on it, and panic if NO price is given (a price-less reserved declaration is a meaningless no-op that would pollute the manifest);
• duplicate metric name → panic (unchanged).

ms.Record(ctx, name, value) continues to REJECT (return an error for) a reserved infra.*/platform.* name: a module may declare its price but never self-report the platform-measured quantity.

BREAKING

The metric kind moves from a positional argument to an option. The exported ms.Kind type and the ms.Counter/ms.Gauge Kind constants are gone; ms.Counter/ms.Gauge are now ms.MetricOptions. VERSION bumped 0.2.2 → 0.2.3.

Deferred

Platform-side enforcement of the override + default cost × 1.2 for platform-infra metrics lands with PR #10.

Design ref

docs-temp/milestone-d-meter/design.md §3a (build rule 3), §4 Axis 1, §5 ("app-module-sdk") — PR #1b.

Review follow-ups (non-blocking, from go review)

• Decl carries an unexported kindDup field; added a doc note that it must be constructed via DeclFromOptions, not a struct literal (a hand-built Decl would skip the conflicting-kind guard). Addressed in this PR.
• meter/meter_test.go reserved-with-kind/unit subtests spread a local []MetricOption into DeclFromOptions — correct, slightly unusual; no change.
• internal/core/resources.go manifest write sets Kind: string(d.Kind); for a reserved override d.Kind == "" and the MetricDecl.Kind json tag is omitempty, so it is omitted from the wire (correct per spec). No change.

🤖 Generated with Claude Code