codec,text,json: proto2 extensions end-to-end

Plugin iterates file.Extensions and per-message Extensions, emitting
\`pb.register_extension(extendee_desc, {…})\` calls. Each extension
becomes a field descriptor attached to the extendee under two
indices: extensions_by_id (wire-decode lookup) and
extensions_by_full_name (encode + textual emission ordering).

Codec encode_message walks data._extensions after the regular field
loop and calls encode_field through the extension's descriptor.
decode_message routes an unknown tag through decode_extension when
extensions_by_id matches; the helper mirrors the in-line dispatch
on field kind (scalar/enum/message/group, singular/repeated) and
stores into result._extensions[full_name].

Text decoder recognizes bracket syntax \`[pkg.ext_name]\` outside an
Any-target and looks the name up in extensions_by_full_name. Names
that don't resolve (e.g. \`[pkg.GroupField]\` for the CapitalCase
group type instead of the lowercase field name) raise a parse error
per spec. Text encoder emits each set extension as
\`[full.name]: value\` or \`[full.name] { … }\`.

JSON encoder emits set extensions under the bracketed full-name key
(\`"[pkg.ext_name]": value\`). JSON decoder recognizes the same
shape and stores into _extensions; unknown bracket-names stay
silently dropped to match the spec's tolerant behavior for unknown
JSON keys.

Extensions on google.protobuf.* descriptors (file/message/field
option extends) are skipped — those are meta-only and our WKT
module doesn't surface their descriptors.

Conformance baseline (strict --enforce_recommended, protobuf v34.1):
  Binary + JSON suite: 2806 ✓ / 0 failures   (was 1493 ✓ / 1313 skipped)
  Text-format suite:    434 ✓ / 0 failures   (was  416 ✓ /   18 skipped)

100% pass.

text,codec: proto2 group text syntax + closed-enum semantics

Text decoder now resolves a group field reference by the group's
capitalized submessage name (e.g. \`Data\`, \`MultiWordGroupField\`)
and by the ASCII-lowercase fold (\`data\`, \`multiwordgroupfield\`),
in addition to the existing lowercase field-name lookup. The \`:\`
between field and \`{\` is optional for groups (matches the
message/map rule).

Proto2 enums are closed: parse_enum_value now rejects an integer
literal that doesn't map to any declared value. The plugin emits
\`closed = true\` on every proto2 enum descriptor (driven by
protoreflect's IsClosed()); proto3 enums stay open to preserve
forward-compatibility on the wire.

Conformance text-format suite: 16 unexpected failures → 3, all in
the remaining extension-bracketed-group cases.

codec: proto2 groups (SGROUP/EGROUP wire format)

Plugin: detect protoreflect.GroupKind, emit kind='group' in the
field descriptor and a SGROUP opening tag in inline codegen. The
group body bypasses the LEN-prefix path entirely — encoder emits
start-tag, body bytes, end-tag in three slots; decoder calls
pb.codec.decode_group(desc, buf, pos, field_id) which reads inner
tags until the matching EGROUP id.

Wire layer already understood SGROUP/EGROUP for skip_field; the new
decode_group reuses the per-tag dispatch from decode_message but
stops on EGROUP instead of end-of-buffer. Repeated groups bracket
each element with its own SGROUP/EGROUP pair.

Runtime parser now desugars `optional|required|repeated group Name
= id { body }` into (a) a nested message named Name and (b) a
synthetic field of kind=group whose lowercased name is `name` — so
source-parsed proto2 schemas behave the same as build-time codegen.

Text format renders groups under the submessage's capitalized name
(`SingleGroup { ... }`) rather than the lowercase field name,
matching mainline protoc's convention.

Adds a vendored, MessageSet-stripped test_messages_proto2.proto in
test/conformance/proto/ (header comment documents the patch).
10 new luatest cases (group_*) cover singular + repeated + text +
descriptor kind, across both codegen modes. 739 tests pass.