[AZ-621] Phase C: build_pre_constructed seeds c7_inference

Third subtask of AZ-618. Extends airborne_bootstrap.build_pre_constructed
additively with c7_inference (GPU InferenceRuntime). Wraps the existing
inference_factory.build_inference_runtime so a BUILD_TENSORRT_RUNTIME /
BUILD_PYTORCH_FP16_RUNTIME mismatch surfaces a clear operator-facing
AirborneBootstrapError naming BOTH airborne C7 flags plus the consuming
component slug, rather than bubbling up RuntimeNotAvailableError with no
context.

New public const C7_AIRBORNE_BUILD_FLAGS pairs each airborne runtime
with its gating env flag (onnx_trt_ep deliberately omitted — research
only). Tests stub at the factory boundary; real GPU/TensorRT load
remains Tier-2 only (consolidated at AZ-624). AZ-619 and AZ-620 test
files extended with a _stub_c7_inference_builder autouse fixture
mirroring the AZ-620 pattern for _build_c6_*.

18/18 runtime_root unit tests pass.

Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/03_implementation/reviews/batch_92_review.md

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+# Code Review Report — Batch 92 (AZ-621 Phase C: c7_inference)
+
+**Batch**: 92
+**Tasks**: AZ-621 (3 cp) — AZ-618 Phase C
+**Date**: 2026-05-19
+**Verdict**: PASS
+
+## Findings
+
+None.
+
+## Phase Notes
+
+- **Phase 1 (Context)**: Loaded `_docs/02_tasks/todo/AZ-621_pre_constructed_phase_c_c7_inference.md` and the AZ-618 umbrella spec. Mapped batch changes to AZ-621 exclusively.
+- **Phase 2 (Spec Compliance)**:
+  - AC-621.1: `_build_c7_inference` is called from `build_pre_constructed`; `test_ac_621_1_adds_c7_inference` asserts the key and identity to the factory return.
+  - AC-621.2: `_build_c7_inference` wraps `RuntimeNotAvailableError` into `AirborneBootstrapError` whose message includes both `BUILD_TENSORRT_RUNTIME` and `BUILD_PYTORCH_FP16_RUNTIME` (sourced from the new `C7_AIRBORNE_BUILD_FLAGS` const) plus the configured consumer slug. Defence-in-depth test covers the no-consumer-configured path.
+  - AC-621.3: `tests/unit/runtime_root/test_az621_pre_constructed_phase_c.py` exists and contains the AC-621.1 / AC-621.2 cases.
+  - Constraints: no new `BUILD_*` env flag introduced (only a Python-level `tuple` listing already-existing flag names). Implementation is strictly additive — AZ-619 and AZ-620 keys still populated; AZ-619/620 test suites green after stubbing the new `_build_c7_inference` symbol via the same autouse-fixture pattern AZ-620 introduced for `_build_c6_*`.
+- **Phase 3 (Code Quality)**: `_build_c7_inference` is ~25 lines, single responsibility (wrap factory + translate error), preserves cause chain via `raise ... from exc`, mirrors the established `_build_c6_descriptor_index` pattern verbatim. Tests use AAA, monkeypatch at the bootstrap module boundary (no real GPU/TensorRT load), assert message content per AC-621.2's explicit naming requirement.
+- **Phase 4 (Security)**: no external inputs, no subprocess/eval/exec, no secrets. The new error message includes operator-facing flag names and component slugs only.
+- **Phase 5 (Performance)**: composition-time only; one factory call per `build_pre_constructed` invocation. No hot-path change.
+- **Phase 6 (Cross-Task Consistency)**: single-task batch — N/A.
+- **Phase 7 (Architecture Compliance)**:
+  - `runtime_root.airborne_bootstrap` (Layer 5) imports `runtime_root.inference_factory` (Layer 5 sibling). Per the Allowed Dependencies table, Layer 5 may import from Layers 1–4 plus its own sibling composition modules. No layer violation.
+  - The imported symbol `build_inference_runtime` is part of `inference_factory.__all__` — Public API respected.
+  - No new module-level cycle introduced (graph remains `airborne_bootstrap → inference_factory → errors`, no back-edge).
+  - No duplicate symbol introduced across components.
+  - No cross-cutting concern locally re-implemented — error translation is bootstrap-specific (mirrors c6 path) and not a candidate for `shared/`.
+
+## Counts
+
+- Critical: 0
+- High: 0
+- Medium: 0
+- Low: 0