feat(r8): ship keep rules for the generated messages

[bmc08gt]

protobuf-javalite ships no keep rules of its own, so every Android consumer has to write
one. code-android-app carried -keep class * extends GeneratedMessageLite { *; } and the
matching $Builder rule. Between them they pinned 29,633 items — 94% of that app's entire
keep radius — and blocked R8 from optimizing 24,909 methods, 81% of every method it was
forbidden to touch. 150 of the 461 message classes and 1,215 of the 3,800 fields those rules
covered come from this package.

Almost none of that was needed. javalite resolves fields reflectively: the schema built from
newMessageInfo looks up Field objects against the generated <name>_ fields. Builders and
message methods are reached from ordinary call sites, so R8 traces them without a rule.
tink-android already ships exactly this narrower form for its own shaded copy of javalite.

-keepclassmembers class * extends com.google.protobuf.GeneratedMessageLite {
    <fields>;
}

-keepclassmembers does not keep the class, so a message type nothing references is still
removed entirely — the rule only applies to messages that survive on their own merit.

Why it is not scoped to com.codeinc.opencode.gen.**

The well-known types (Any, Timestamp, Duration, Struct) come from protobuf-javalite
itself, and other dependencies ship generated messages with no rules of their own —
firebase-perf is one. A rule scoped to the generated package would leave both broken under
R8 full mode. Measured against the app, the rule's radius spans four artifacts:

Artifact	Fields kept
flipcash2-client-protocol	1,721
ocp-client-protocol	1,193
firebase-perf	138
protobuf-javalite	20

Both contract packages ship identical rule text and R8 collapses them into a single entry, so
the duplication costs nothing and either artifact alone is sufficient.

Effect on the consumer

Built through protoLocalRoot, with the app's own protobuf rules removed:

live methods    230,686 -> 205,144   (-25,542)
live classes     42,338 ->  41,770      (-568)
live fields      96,939 ->  95,692    (-1,247)

obfuscation      89.0% -> 96.1%
optimization     89.3% -> 96.3%
shrinking        89.1% -> 96.1%

Owning the rule here also means an app that adds this dependency gets working protobuf under
minification without knowing any of the above.

Consumer side: code-payments/code-android-app#1370

Release

Ships as 0.3.0. Merged main in and added the ## 0.3.0 changelog section — publish.yml
uses it as the release notes and refuses to publish a version without one — and pointed the
README install snippets at it. No contract change, so the Swift tag carries nothing new.