Introduce MergePolicy (AUTHORITATIVE/ENRICHMENT/FALLBACK), FieldGroup enums,
MergeReport diagnostics, DiscoveryLayer interface and LayerOutput struct.
Implement MergePipeline core with single-layer passthrough and multi-layer
field-group merge with conflict resolution.

…components, docs

Fix per-field-group owner tracking in MergePipeline::merge_entities().
Previously, all field groups used the first layer as comparison target
for subsequent merges. Now each field group tracks its current owning
layer independently, preventing lower-priority layers from incorrectly
overriding authoritative values won by middle layers (e.g., Plugin
ENRICH overriding Runtime AUTH for STATUS).

Restore create_synthetic_components config check in discover_components()
so that setting it to false produces per-node components (legacy mode)
instead of unconditionally grouping by namespace.

Update manifest-schema.rst and manifest-discovery.rst to document
path-segment-boundary namespace matching semantics (was documented as
"exact match" but code uses prefix matching with segment boundaries).

- Fix YAML param naming mismatch: use dot-notation (discovery.mode,
  discovery.manifest_path) matching gateway_params.yaml structure
- Make hybrid mode fail on missing/invalid manifest (same as manifest_only)
- Add WARN logs for unknown discovery.mode enum values
- Fix stale param references in tutorials and config docs
- Remove stale expose_nodes_as_apps from discovery-options.rst
- Add unit tests for FieldGroup/MergePolicy parsing and layer policy overrides
- Add integration tests for gap-fill, namespace filter, layer policies,
  and legacy discovery mode
- Update existing integration tests with corrected param names

Each integration test now gets a unique GATEWAY_TEST_PORT from CMake
(stride of 10, starting at 9100), eliminating port conflicts during
parallel CTest execution. All tests use get_test_port() consistently
instead of hardcoded ports.

Also fixes the 4 new discovery integration tests:
- Correct poll_endpoint_until lambda pattern (return data, not bool)
- Use collection response format (data['items'])
- Add explicit timeouts for discovery-dependent polls
- Fix server.port param name in create_gateway_node helper

…spection, safety

- Fix data race in HybridDiscoveryStrategy: get_linking_result() now
  reads from cached_result_ (protected by mutex) instead of pipeline_
  internals that execute() mutates without synchronization
- Eliminate double discover_apps() in RuntimeLayer: apps are discovered
  once and passed to discover_components() via new overload
- Add provides_runtime_apps() virtual to DiscoveryLayer, replacing
  fragile name-based "runtime" check that a plugin could collide with
- Use .at() instead of operator[] in RuntimeLinker binding-conflict
  warning to prevent accidental map insertion
- Add missing #include <cctype> in plugin_layer.cpp for std::isalnum