feat: fetch() with outputFields parameter (closes #176)

[s2x]

Description

Closes #176

Adds optional output-field selection to ZVecCollection::fetch(), matching upstream zvec v0.5.0+ (alibaba/zvec#358) and the Go SDK's Fetch(pks, FetchOptions).

Changes

• FFI (ffi/zvec_ffi.h, zvec_ffi.cc, zvec_ffi_php.h): zvec_collection_fetch extended with output_fields / output_field_count parameters; builds std::optional only when count > 0 (nullopt = all fields, exact upstream semantics from c_api.cc). Per-entry null guard added.
• PHP (src/ZVec.php): fetch(array|string ...$args) — BC-compatible dual form: legacy fetch('a', 'b') and new fetch(['a','b'], ['name','score']). Full input validation via ZVecException (empty/non-string PKs, non-array/non-string/empty output fields, extra arguments).
• Test: tests/test_fetch_output_fields.phpt — 8 scenarios: subset selection, BC all-fields, array form, unknown-fields-ignored, 3 validation throws, extra-args throw.
• Docs: README API listing + CHANGELOG updated.

Testing

• Builds locally (./build_ffi.sh clean on macOS/arm64)
• All .phpt tests pass: 175 passed, 0 failed, 2 expected-fail, 1 skipped
• No test database leftovers (test_dbs/ empty)

Code Review

• Passed subagent code review (2 low-severity findings — both fixed)
• Re-review approved ("Code looks good, no issues to fix")
• Follow-up candidates reported (includeVector exposure, nullable normalization — separate issues)

Notes

• include_vector intentionally not exposed (out of scope for fetch() with outputFields parameter (zvec v0.5.0) #176): vector fields are always present in fetched docs, matching the upstream default.
• Unknown output field names are silently ignored by the C++ layer (documented in test; upstream Node/Python behave the same).