Add PCD nearest neighbor search demo

[qiyu-lu]

Summary

• Add a PCD-based nearest-neighbor search demo for likd-tree
• Visualize the source cloud, query point, nearest point, and connecting line
• Add brute-force validation and terminal timing output
• Document the demo usage in README

Test

• cmake -B build
• cmake --build build
• ./build/nearest_search_pcd_demo ./test/pcd/globalMap.pcd 0.9 0.1 0
• Brute-force check: MATCH

[Copilot]

Pull request overview

Adds a new PCL/PCD-based nearest-neighbor demo to help users visualize and validate likd-tree queries, and wires it into the build + documentation.

Changes:

• Add nearest_search_pcd_demo that loads a PCD, builds a KDTree, queries a point, brute-force validates, and optionally visualizes the result.
• Update CMake to only build the new demo when PCL visualization is available (while still supporting PCL common/io when visualization is missing).
• Document usage in README.md and update .gitignore.

Reviewed changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated 2 comments.

File	Description
test/nearest_search_pcd_demo.cpp	New PCD nearest-neighbor demo with optional PCLVisualizer rendering and brute-force validation.
README.md	Adds usage docs, example output, and a screenshot for the new demo.
CMakeLists.txt	Detects PCL visualization component and conditionally builds/link the new demo.
.gitignore	Adds ignores for /.cache/ and test/pcd/*.pcd.

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