bench,codec: pin proto2 paths, kill pairs() on extension hot path

JIT trace gate gains 12 proto2-specific checks (required encode/decode,
groups singular + repeated, extension encode/decode). The extension-encode
check fired NYI on bytecode 72 (ISNEXT) — pairs() over the new
extensions_by_full_name hash is the same trace-abort that gates map
encode. Fix: register_extension now also appends to extensions_list,
an array view. codec.encode_message, text.emit_message and
json.encode_message all switch to ipairs over the list. Hash tables
stay around for O(1) lookups in decode (extensions_by_id) and bracket-
name resolution (extensions_by_full_name).

Inline (full-mode) codegen learns to walk extensions too: before this
commit the inline encoder skipped t._extensions entirely (only the
runtime codec walked it), so a generated _encode silently dropped any
extension data set on the message. Add the array-walk after the field
loop and a decode_extension dispatch in the unknown-tag branch — the
inline path now matches runtime byte-for-byte.

bench/bench.lua parameterizes over a FIXTURES list so the baseline can
cover both hello.Person and a new proto2_basic.BenchPayload fixture
(required + group + extension + repeated). baseline.json restructured
under "schemas": [{schema, results}, …]; compare walks both. Fresh
numbers committed.

740/740 luatest cases pass; JIT gate 37/37; conformance 2806 binary+JSON
and 434 text-format, both 0 failures.

codegen: preserve descriptor options as `options = { ... }`

Every populated *Options message — FileOptions, MessageOptions,
FieldOptions, OneofOptions, EnumOptions, EnumValueOptions,
ServiceOptions, MethodOptions — surfaces on the generated descriptor as
a plain Lua sub-table named `options` (or `oneof_options` /
`value_options` for the per-member shapes). Standard fields use their
proto name as a bare Lua key; extensions use their fully-qualified
extension name as a bracket-quoted string key.

The walker is generic — no extension-specific code paths. Consumers
pull whatever they care about: `(google.api.http)` for REST routing,
`(versionpb.etcd_version_*)` for compatibility gates, `[deprecated =
true]` for migration tooling, and any in-house extension without pb
knowing about them. Standard fields sort alphabetically before
extensions (also alphabetical by full name) so codegen output stays
byte-identical across runs.

The `options` key is only emitted when at least one field is populated,
so option-free protos produce zero-diff output to before. Resolver
re-links *Options* messages so in-file extensions surface via the
protoreflect walker — protogen builds f.Desc before in-file extensions
are registered, and only f.Proto gets the post-pass fix-up, so we
rebuild the resolver manually.

UninterpretedOption is treated as a codegen-time error: a populated
entry means protoc couldn't resolve the extension, and emitting
opaque parser state would hide the problem.

docs: full reference + how-to set, migrate Makefile to Justfile

Documentation overhaul that adds the missing user-facing surface:
four reference pages (runtime-api, generated-api, cli, grpc-contract),
twelve how-tos walking from first-message through custom transports,
a troubleshooting page, and a docs/index map. Every how-to references
a runnable artifact under examples/, all of them verified end-to-end.

Build system migration: the Makefile is gone; the Justfile is now
the canonical entry point and absorbs every target. examples/Justfile
ships one recipe per runnable example, forwarded via top-level
'just examples <name>'. The 'examples are part of the documented
surface' convention is pinned in CLAUDE.md, alongside a dedicated
section on updating the conformance harness (PROTOBUF_TAG bumps,
libjsoncpp path drift, new test-category wiring).

Stale-number sweep across README/PLAN/CLAUDE: fixture count 18→10,
test count 613/130→639, wire.lua LOC dropped, M7 marked done.
Descriptor-shape block deduplicated against codegen.md as the
canonical source. gRPC transports spec status reframed from
'draft / decision deferred' to 'shipped contract; external
transports deferred'.

.gitignore picks up *.snap / *.xlog / *.vylog / *.run / *.pid /
512.lock so example state can't leak into the working tree.