docs: install path, prefix= plugin param, int64 migration patterns

* README: add an "Install" section recommending `tt rocks make` from a
  local clone until the canonical remote is published, plus a note in
  the rockspec flagging source.url as aspirational (the URL today
  resolves to "no such repository").
* README: document the `--tarantool_opt=prefix=<path>` plugin parameter
  alongside `(tarantool.lua_package)` — the on-disk + require-string
  semantics and how the two compose were previously only discoverable
  from main.go and docs/howto/02-module-layout.md.
* README: add "Migrating from a Lua proto library that auto-down-casts
  int64" listing the four places cdata int64 needs a tonumber()
  wrapper (log format verbs, numeric for-loop bounds, string.format,
  table keys) and the 2^53 precision caveat.

build: add scm-1 rockspec for the pb runtime

Installs the 12 modules under pb.* from runtime/pb/ via builtin build
type; lua >= 5.1 is the only declared dependency since every other
import (bit, datetime, digest, ffi, fiber, json, protobuf, utf8) is a
Tarantool builtin. Plugin stays out of scope — it's a Go binary built
via the Justfile, not a luarocks artifact. Source URL points at the
planned sourcecraft.dev home that matches go.mod's module path.