[AZ-527] Consolidate _assert_engine_output_dim into c2-internal helper

Closes cumulative review batches 49-51 Finding F1 (Medium /
Maintainability) -- the 7-way duplication of _assert_engine_output_dim
across c2_vpr secondary VPR strategy modules.

Add c2-internal helper assert_engine_output_dim(inference_runtime,
handle, preprocessor, descriptor_dim, *, output_key='embedding',
input_key='input') in src/gps_denied_onboard/components/c2_vpr/
_engine_dim_assertion.py. The helper runs a zero-init dry-run
inference at preprocessor.input_shape() and asserts the engine output
dict carries (1, descriptor_dim) under output_key. Raises
gps_denied_onboard.config.schema.ConfigError on mismatch (preserving
the prior error envelope and message wording byte-identically).

Migrate 7 strategy modules (ultra_vpr, net_vlad, mega_loc, mix_vpr,
sela_vpr, eigen_places, salad) to import the helper and delete the
local _assert_engine_output_dim definitions + their inline
'AZ-527 (planned)' comments. NetVLAD is the only call site that
overrides output_key='vlad_descriptor'; the other 6 explicitly pass
output_key=_OUTPUT_KEY + input_key=_ENGINE_INPUT_KEY (matching helper
defaults but documenting strategy contract at the call site).

Add tests/unit/c2_vpr/test_az527_engine_dim_assertion.py (14 tests,
AAA pattern, Protocol-conforming fakes) covering AC-1..AC-4: helper
signature; wrong shape raises ConfigError naming both dims; missing
output key raises ConfigError naming the missing key; AST-walk
regression guard for stray definitions outside the helper module
(modeled on AZ-526's test_ac4_az526_no_module_level_iso_ts_from_clock_outside_helper);
import-grep regression guard verifying all 7 strategy modules import
the helper.

AC-5 (existing AZ-337/338/339/340 AC-6 sub-tests pass unmodified) is
exercised transitively: c2_vpr/ full directory 230/230 PASS, no test
file modified outside the new test_az527_*. AC-6 (AZ-270 + AZ-507
layer lints) verified by tests/unit/test_az270_compose_root.py
8/8 PASS.

Code-review verdict: PASS (zero findings). Ruff clean.

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

src/gps_denied_onboard/components/c2_vpr/mega_loc.py

@@ -59,6 +59,9 @@ from gps_denied_onboard._types.inference import (
 )
 from gps_denied_onboard._types.vpr import VprQuery, VprResult
 from gps_denied_onboard.clock import Clock
+from gps_denied_onboard.components.c2_vpr._engine_dim_assertion import (
+    assert_engine_output_dim,
+)
 from gps_denied_onboard.components.c2_vpr._faiss_bridge import FaissBridge
 from gps_denied_onboard.components.c2_vpr._preprocessor_mega_loc import (
     MegaLocBackbonePreprocessor,
@@ -393,7 +396,14 @@ def create(
         clock=clock,
     )
 
-    _assert_engine_output_dim(inference_runtime, handle, preprocessor)
+    assert_engine_output_dim(
+        inference_runtime,
+        handle,
+        preprocessor,
+        DESCRIPTOR_DIM,
+        output_key=_OUTPUT_KEY,
+        input_key=_ENGINE_INPUT_KEY,
+    )
 
     logger.info(
         "C2 VPR strategy ready",
@@ -421,31 +431,3 @@ def create(
     )
 
 
-def _assert_engine_output_dim(
-    inference_runtime: InferenceRuntimeCut,
-    handle: EngineHandle,
-    preprocessor: MegaLocBackbonePreprocessor,
-) -> None:
-    # The 4-way duplication of this helper (ultra_vpr / net_vlad /
-    # mega_loc / mix_vpr) will be consolidated by AZ-527 (hygiene
-    # PBI sized in parallel with AZ-339 land). The duplication is
-    # intentional for now: extracting earlier would expand AZ-339's
-    # scope past the two new strategies.
-    h, w = preprocessor.input_shape()
-    probe = np.zeros((1, 3, h, w), dtype=np.float16)
-    outputs = inference_runtime.infer(handle, {_ENGINE_INPUT_KEY: probe})
-    if _OUTPUT_KEY not in outputs:
-        raise ConfigError(
-            f"engine output shape mismatch: {_OUTPUT_KEY!r} key absent; "
-            f"got keys {sorted(outputs.keys())!r}"
-        )
-    actual = np.asarray(outputs[_OUTPUT_KEY])
-    if (
-        actual.ndim != 2
-        or actual.shape[0] != 1
-        or actual.shape[1] != DESCRIPTOR_DIM
-    ):
-        raise ConfigError(
-            f"engine output shape mismatch: expected (1, {DESCRIPTOR_DIM}), "
-            f"got {tuple(actual.shape)}"
-        )