fix: improve base64 and decodeUTF8 error messages

[He-Pin]

Summary

• Fixes base64 error classification to use UTF-8 byte length instead of UTF-16 code-unit length.
• Keeps the UTF-8 byte-length heuristic allocation-free instead of calling getBytes on the error path.
• Restores decodeUTF8 element-index reporting and uses stable numeric rendering for array element values.
• Adds a raw-byte fast path for decodeUTF8; otherwise validates and fills the byte array in a single indexed pass.
• Moves renderer long scratch buffers from shared companion state to renderer instances so independent interpreters do not share mutable scratch memory.
• Restores the agent-facing threading-model note in CLAUDE.md.

Current head: b794c651c7bc8416d3e66fdf752f3563c0632854

Performance Notes

• Base64 string classification scans chars once and does not allocate a temporary UTF-8 byte array.
• decodeUTF8 avoids iterator/zipWithIndex/map pipelines in the validation path.
• The byte-array-backed path decodes directly from rawBytes.
• Renderer scratch buffers are one tiny allocation per renderer instance, not shared global mutable state and not per number.

Verification

• rtk ./mill --no-daemon 'sjsonnet.jvm[3.3.8].reformat'
• rtk ./mill --no-daemon 'sjsonnet.jvm[3.3.8].checkFormat'
• rtk ./mill --no-daemon 'sjsonnet.js[2.13.18].checkFormat'
• rtk ./mill --no-daemon 'sjsonnet.jvm[3.3.8].test' sjsonnet.Base64Tests
• rtk ./mill --no-daemon 'sjsonnet.jvm[3.3.8].test' sjsonnet.StdStringTests
• rtk ./mill --no-daemon 'sjsonnet.jvm[3.3.8].test' sjsonnet.FileTests
• rtk ./mill --no-daemon 'sjsonnet.js[2.13.18].test' sjsonnet.Base64Tests
• rtk ./mill --no-daemon 'sjsonnet.js[2.13.18].test' sjsonnet.StdStringTests
• rtk ./mill --no-daemon 'sjsonnet.js[2.13.18].test' sjsonnet.FileTests
• rtk ./mill --no-daemon 'sjsonnet.wasm[2.13.18].test' sjsonnet.Base64Tests
• rtk ./mill --no-daemon 'sjsonnet.wasm[2.13.18].test' sjsonnet.StdStringTests
• rtk ./mill --no-daemon 'sjsonnet.wasm[2.13.18].test' sjsonnet.FileTests
• rtk ./mill --no-daemon 'sjsonnet.native[2.13.18].compile'
• Qoder stdout review of the final diff: no must-fix findings.