[andrewtdiz]

Part of #793.

Summary:

• Recognize utf16le, utf-16le, ucs2, and ucs-2 in static Buffer encoding folding and runtime encoding parsing.
• Encode Buffer.from(string, encoding) UTF-16LE aliases as little-endian UTF-16 byte pairs.
• Decode buf.toString(encoding) UTF-16LE aliases and reuse the same string-to-bytes path for Buffer search string needles.

Verification:

• DeepWiki: reference/deepwiki/nodejs-node-buffer-utf16le-search-encoding-2026-05-28.md.
• Baseline exact node-suite/buffer/search/ucs2-and-buffer-needle: COMPILE FAIL, report test-parity/reports/parity_report_20260528_122044.json.
• Final exact node-suite/buffer/search/ucs2-and-buffer-needle: PASS, report test-parity/reports/parity_report_20260528_122510.json.
• Full node-suite/buffer/search: PASS 7/7, report test-parity/reports/parity_report_20260528_122551.json.
• Full granular node-suite/buffer: 104 PASS / 12 parity FAIL / 0 compile FAIL, report test-parity/reports/parity_report_20260528_122604.json; target is green and the prior compile residual is gone.
• cargo check -p perry-runtime -p perry-stdlib -p perry-codegen: PASS with existing warnings.
• cargo fmt --check, jq empty test-parity/known_failures.json, and git diff --check: PASS.

Non-goals:

• No buf.write(..., "utf16le") implementation.
• No unknown-encoding error-shape cleanup.
• No broad string-decoder/transcode work.
• No unrelated Buffer coercion cleanup.