fix(core): make @ambarltd/core/tracing resolve — promote facade to file-module

[christopherpcosta]

Problem

@ambarltd/core/tracing does not resolve from the built package — caught while preparing the 0.1.17 release (0.1.17 is not yet published, so nothing shipped broken).

tracing was the package's first directory-index module (src/tracing/index.ts → dist/tracing/index.js). The exports wildcard substitutes subpaths literally with no index fallback:

"./*" → "./dist/*.js"   ⇒   "@ambarltd/core/tracing" → dist/tracing.js   (did not exist)

So import { setTracer } from "@ambarltd/core/tracing" failed with MODULE_NOT_FOUND, while …/tracing/opentelemetry happened to work. HartAgency#173 imports both.

Fix — follow the package's own module convention

Every public module in core is a top-level file resolved by the single wildcard (future, router, maybe, json/schema, json/decoder, …; consumer base has zero directory-index imports). So instead of adding a special-case exports entry (first version of this PR), the facade moves to where the convention puts it:

• src/tracing/index.ts → src/tracing.ts (file-module facade, like future.ts)
• src/tracing/ keeps the implementations (opentelemetry, proxy, simple) — the same namespace shape as json/
• 4 relative-import line updates; package.json untouched — the wildcard stays the only export rule

Public import paths are exactly as intended and unchanged: @ambarltd/core/tracing and @ambarltd/core/tracing/opentelemetry.

Validation (against the built artifact)

require.resolve('@ambarltd/core/tracing')               → dist/tracing.js              ✅ (was MODULE_NOT_FOUND)
require.resolve('@ambarltd/core/tracing/opentelemetry') → dist/tracing/opentelemetry.js ✅
require.resolve('@ambarltd/core/router')                → dist/router.js               ✅ (wildcard untouched)
dist/tracing.js exports: event, setTracer, trace, traceF, traceP                       ✅
tsc build clean · typecheck clean · core tests 123/123 pass                            ✅

Release note

No version bump needed — 0.1.17 (already on main, unpublished) ships this. After merge: cd core && pnpm publish (manual — the ambar-core workflow is CI-only).

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