fix: skip byte arrays in shadow mismatch JSON

[lan17]

Summary

Follow-up to #113. Byte arrays are the one shape left where the mismatch JSON preview is actively counterproductive.

Native JSON expands a Buffer or other Uint8Array view into one decimal element per byte, so an 8 KiB-clamped preview becomes a run of digits:

{"blob":{"type":"Buffer","data":[7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,...

That is ~6 characters of JSON per byte of payload: the worst cost-per-byte of any shape, and it crowds every other field out of the capped preview. Measured before this change, a record with a 200 KB Buffer field produced ~2 MB of intermediate JSON to emit 8192 characters of the digit 7.

After, at any depth:

{"id":"1","blob":"<binary 200000 bytes>"}

Implementation

Buffer.prototype.toJSON runs before a JSON.stringify replacer, which splits the handling:

• A plain Uint8Array view reaches the replacer intact and is swapped for the marker before expansion, so no per-byte array is built.
• A Buffer has already been converted to { type: "Buffer", data: [...] } by the time the replacer sees it, so it is recognized from that result. This keeps the array out of the intermediate JSON string, but the toJSON allocation itself is still paid — the README notes that limit rather than implying otherwise.
• A top-level view is checked before JSON.stringify so a bare binary cached value skips toJSON entirely.

A plain record already shaped like { type: "Buffer", data: [...] } is reduced the same way. That is deliberate: native JSON renders a real Buffer identically, so collapsing the shape loses no information the previous output preserved.

Scope

No public API change. shadow-log-json.ts is internal and not exported from the root entry, so this alters only the contents of an opt-in logMismatches warning. No new config, metric, label, or dependency. The 2 KiB key cap, 8 KiB value caps, ...[truncated] marker, and null-on-unserializable behavior are untouched.

Validation

Node 26.5.0, from main@34bd022:

• pnpm typecheck — clean
• pnpm test — 409 passed (4 new cases covering nested and top-level views, offset/pooled views, large-buffer crowding, and the look-alike record)
• pnpm build — clean; verified the marker is present in dist/index.js and the CommonJS bundle loads
• pnpm test:package — not run locally (corepack unavailable in my environment); it exercises the packed public surface, which this change does not touch. Worth confirming in CI.

🤖 Generated with Claude Code