fix(node:buffer): estimate base64 byte length

[andrewtdiz]

Part of #793.

Summary

• Match Node's Buffer.byteLength(str, "base64"|"base64url") estimate instead of decoding first.
• Count UTF-16 code units for string inputs, discount up to two trailing = padding characters, and apply the 3/4 base64 ratio.
• Keep Buffer.from/base64 decode behavior, invalid-base64 validation, and non-base64 encodings out of scope.

Evidence

• Baseline exact node-suite/buffer/byte-length/edge-encodings: FAIL, report test-parity/reports/parity_report_20260528_110221.json.
• Final exact node-suite/buffer/byte-length/edge-encodings: PASS, report test-parity/reports/parity_report_20260528_110601.json.
• Full node-suite/buffer/byte-length: PASS 6/6, report test-parity/reports/parity_report_20260528_110654.json.
• Full granular node-suite/buffer: 104 PASS / 11 parity FAIL / 1 compile FAIL, report test-parity/reports/parity_report_20260528_110702.json; byte-length is green, with allocation/concat residuals covered by open PRs fix(node:buffer): reject negative allocation size #2274/fix(node:buffer): honor concat total length #2271.
• 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.

DeepWiki note saved locally at reference/deepwiki/nodejs-node-buffer-byte-length-base64-edge-2026-05-28.md (not staged).

Non-goals

• No Buffer.from base64 decode semantics.
• No invalid-base64 validation.
• No hex/ascii/utf16 byteLength changes.
• No broad buffer inventory cleanup.