docs: record spanenc in adoption boundaries and cross-link from gcvctor

[apstndb]

Problem

spanenc now exists as the client-semantics layer over gcvctor: reflection-based Go value → GCV conversion mirroring the client's internal encodeValue / structToMutationParams / spanner struct-tag semantics (with structfields hosting the Apache-2.0 fork of internal/fields). spanvalue's docs predate it:

1. AGENTS.md "Adoption boundaries" lists string→GCV parsing (spanbridge) and PG table cells (spanpg), but not the reflection/client-tag-semantics boundary. Without it, future feature requests like "make gcvctor accept arbitrary Go values / struct tags" have no recorded answer. Suggested line: reflection-based, client-encodeValue-compatible Go value → GCV (struct tags, Null* wrappers, spanner.Encoder, typed-NULL inference) lives in spanenc; gcvctor stays explicit, strict constructors with caller-supplied types.
2. gcvctor doc.go / README discoverability: a one-line pointer ("for converting arbitrary Go values with the official client's encoding semantics, see github.com/apstndb/spanenc") mirrors the existing pointer to the root spanvalue package and helps pkg.go.dev visitors find the right layer.

Both changes are documentation-only.

Context

spanenc deliberately reuses gcvctor for all GCV construction (strictness invariants: no nil Types, typed NULLs, strict ARRAY/STRUCT assembly) and documents its divergences from the client on its side. Keeping the boundary recorded on the spanvalue side closes the loop.

[apstndb]

Priority raised (spanenc downstream). Documentation-only; unblocks clear adoption-boundary messaging for spanenc consumers. Docs PR incoming.