observability: phase 6.1 PR-C conformance fillin

Drive four spec observability conformance fixtures end-to-end
through the existing test_observability harness:

- 002-otel-subgraph-hierarchy: §4.5 synthetic dispatch span
  parents under invocation; inner-node spans parent under
  dispatch span with the nested ["outer_sub", "inner_x"] /
  ["outer_sub", "inner_y"] namespace.
- 003-otel-error-status: §4.2 ERROR status mapping for the
  node_exception case (status_description == "node_exception",
  recorded exception event, openarmature.error.category
  attribute, ok_node stays OK).
- 007-otel-retry-attempt-spans: both sub-cases. Three sibling
  attempt spans with distinct attempt_index 0..2, single shared
  invocation parent. ERROR/ERROR/OK on retry-succeeds-third;
  ERROR x3 on retry-exhausts.
- 011-otel-determinism: signature-tuple comparison
  (name, status_code, sorted attrs minus the canonical
  non-deterministic-by-design ignore set: invocation_id +
  auto-generated correlation_id) across two runs.

Two minor YAML-shape translations kept inline in the
fixture-specific drivers so the existing adapter stays
unchanged (the adapter is shared with other suites; this PR
doesn't touch it):

- Fixture 007 flaky.fail_count=N rewritten to the adapter's
  failure_sequence shape before build_graph.
- Fixture 011 when:{field, gt: 0} edge syntax rewritten to the
  adapter's condition:{if_field, equals, then, else} shape via
  _translate_011_when_edges (gt:0 with the fixture's
  deterministic input is functionally equivalent to equals:1
  because counter==1 always under that input).

Map updates:

- _SUPPORTED_FIXTURES grows from 4 to 8.
- _DEFERRED_FIXTURES shrinks from 7 to 3 (004 / 006 / 010);
  each remaining note now spells out the specific gating-PR +
  spec/architectural rationale (PR-C.1 + proposal 0012 /
  PR-C.2 / PR-C.3) rather than the generic "deferred to
  Phase 6.1" placeholder.

Module docstring rewritten to distinguish the 8 driven
fixtures from the 3 deferred ones.

No engine, observer, or correlation changes. Pure test-side
work.

392 tests pass (was 388; net +4 from the new fixture drivers).
5 skipped (3 deferred conformance fixtures + 2 pre-existing).
Pyright clean.

Author: chris-colinsky