enhancement: fetch() mixing scalar PK and outputFields array fails with confusing "PKs must be non-empty strings"

[s2x]

Finding (from #192 follow-up)

fetch('doc1', ['name']) (scalar PK followed by an outputFields array) fails with PKs must be non-empty strings — technically correct but gives no hint about the actual calling convention.

Evidence

• src/ZVec.php:766-776 — in the scalar form, all of $args are treated as PKs, so the array element fails the string check.
• The two supported forms are fetch('pk1', 'pk2') and fetch(['pk1','pk2'], ?array $outputFields) — mixing scalar PK + array outputFields is not supported.
• Verified by subagent review (CONFIRMED; no duplicate — closed fetch() with outputFields parameter (zvec v0.5.0) #176 is unrelated).

Suggested improvement

Detect an array in the scalar-form arguments and throw (or better: accept) with a message pointing to the calling convention, e.g.:
"Mixing scalar PKs with an outputFields array is not supported — use fetch(['pk1'], ['name']) instead" (or support the form outright).

Low priority / DX polish.