codegen: emit strict <Type>_fields / _oneofs constants for lazy view

Lazy-view callers passing a typo'd field name to :get / :has / :set /
:clear / :which got `nil` back, indistinguishable from a legitimately-
absent optional field. Failures surfaced as missing data downstream.

Each generated message now exports a M.<Type>_fields table mapping
each field name to itself (and M.<Type>_oneofs for oneof groups),
wrapped by a new pb.field_names() helper that errors on unknown-key
reads and on any write. Routing field-name arguments through these
tables turns a typo into a load-time error at the read site.

Eager _encode / _decode keep round-tripping plain Lua tables — the
constants table is a lazy-view contract, documented in
docs/api-modes.md. README and lazy_test.lua converted to the new
pattern; three new tests cover typo / read-only / oneof-typo errors.

lazy: zero-copy decode view (decode_lazy) with passthrough re-encode

Adds pb.decode_lazy(desc, bytes) returning a MessageView/ArrayView/MapView
that indexes the wire bytes in a single pass and decodes individual
fields only on :get / :at access. Nested messages return more lazy
sub-views; WKT descriptors (those carrying desc.decode) are
eager-wrapped so the API stays uniform.

Surface (see runtime/pb/lazy.lua):
  - MessageView: :get / :has / :which / :iter / :names / :set / :encode
  - ArrayView:   :len / :at / :iter / :tolist
  - MapView:     :get / :has / :keys / :iter / :totable

:encode is three-modes: WKT delegates to desc.encode on the materialized
table; untouched views return their original bytes verbatim
(passthrough); mixed views walk fields in id order, splicing clean
segments and re-emitting dirty ones. Sub-MessageView mutations
propagate to parent encode via a flat _sub_msg_views array (walked with
ipairs, so :is_dirty stays on a single JIT trace — pairs over a hash
is NYI in LuaJIT 2.1).

Codegen emits M.<Type>_decode_lazy in both modes as a one-line
delegation to pb.decode_lazy(<desc>, b); no inline expansion.
codec.encode_field is exposed so the lazy passthrough emitter can
splice fresh bytes for a single dirty field without rebuilding the
whole message.

Tests: 40 new lazy_test.lua cases parameterized over both codegen
modes; all 11 interop fixtures round-trip byte-equal through
decode_lazy(b):encode() in both modes. Total: 300/300 luatest, up
from 226.