fix(node:buffer): handle Buffer.from object sources

[andrewtdiz]

Part of #793.

Summary:

• detect plain object array-like inputs in Buffer.from before the ArrayHeader fallback
• require a numeric length, return empty for non-number/non-positive lengths, and truncate fractional lengths
• honor custom valueOf() string/object results and string-returning Symbol.toPrimitive sources
• read indexed properties through object lookup and share byte coercion with array sources

Verification:

• DeepWiki: reference/deepwiki/nodejs-node-buffer-from-arraylike-coercions-2026-05-28.md
• DeepWiki: reference/deepwiki/nodejs-node-buffer-from-object-primitives-2026-05-28.md
• Baseline exact node-suite/buffer/from/arraylike-coercions: FAIL, report test-parity/reports/parity_report_20260528_125501.json
• Final exact node-suite/buffer/from/arraylike-coercions: PASS, report test-parity/reports/parity_report_20260528_125818.json
• Baseline exact node-suite/buffer/from/valueof-symbol-to-primitive: FAIL, report test-parity/reports/parity_report_20260528_130501.json
• Final exact node-suite/buffer/from/valueof-symbol-to-primitive: PASS, report test-parity/reports/parity_report_20260528_130903.json
• Full node-suite/buffer/from: 15 PASS / 1 parity FAIL, report test-parity/reports/parity_report_20260528_130955.json; only primitive input validation remains red and is covered by fix(node:buffer): reject primitive from inputs #2287
• Full node-suite/buffer: 105 PASS / 10 parity FAIL / 1 compile FAIL, report test-parity/reports/parity_report_20260528_131022.json; targets green
• cargo check -p perry-runtime -p perry-stdlib -p perry-codegen: PASS with existing warnings
• cargo fmt --check: PASS
• jq empty test-parity/known_failures.json: PASS
• git diff --check: PASS

Non-goals:

• no primitive input validation, covered by fix(node:buffer): reject primitive from inputs #2287
• no non-string Symbol.toPrimitive TypeError cleanup
• no ArrayBuffer offset validation
• no broad Buffer.from error-message cleanup