fix(otel): make lite-bootstrap[fastmcp] importable (api/sdk split)

[lesnik512]

Summary

lite-bootstrap[fastmcp] (and any environment with opentelemetry-api but not opentelemetry-sdk) crashed on import lite_bootstrap. Full rationale: planning/changes/2026-07-19.01-fix-otel-api-sdk-conflation.md.

Reproduced on regular (GIL) CPython 3.12:

$ uv pip install "lite-bootstrap[fastmcp]"; python -c "import lite_bootstrap"
  File ".../instruments/opentelemetry_instrument.py", line 18, in <module>
    from opentelemetry.sdk import resources
ModuleNotFoundError: No module named 'opentelemetry.sdk'

Root cause: is_opentelemetry_installed = find_spec("opentelemetry") is True with only opentelemetry-api, but the OTel instrument imports opentelemetry.sdk.*. fastmcp pulls bare opentelemetry-api transitively. (This is the third of three incomplete-opentelemetry-stack bugs; the other two shipped in #153.)

What changed

• Add import_checker.is_opentelemetry_sdk_installed = _safe_find_spec("opentelemetry.sdk").
• Gate the instrument's sdk-import block and the pyroscope-processor block on the new flag; check_dependencies() requires both api and sdk. The six api-only consumers (logging trace-injection, framework get_tracer_provider, faststream health-check spans) stay on the api flag.
• bootstrap() now warns (InstrumentDependencyMissingWarning) instead of silently skipping when opentelemetry_endpoint is set but the gRPC OTLP exporter package is absent.
• Promotes architecture/instruments.md (three-way api/sdk/exporter guard) and clears the OTel-stack dependency-model deferral. deferred.md fastmcp entry updated — now imports on 3.14t (cffi still blocks 3.13t).

Testing

• TDD: two regression tests (sdk-absent module reimports cleanly + check_dependencies False; endpoint-without-exporter warns). Both provably fail on pre-fix code.
• just test: 211 passed, 100% coverage. just lint-ci clean (ty local + latest).
• Verified uv pip install ".[fastmcp]" + import lite_bootstrap succeeds on regular 3.12.
• Independent review (reproduced both crashes via worktree revert): ready to merge.

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