codegen: add a table-driven codec strategy

[iainmcgin]

Adds CodecStrategy::Table, the generator for the table codec proposed in #463. Unrolled stays the default. Stacked on #468 (the buffa::table runtime), which is stacked on #467.

buffa_build::Config::new()
    .codec_strategy(CodecStrategy::Table)
    .codec_strategy_in(CodecStrategy::Unrolled, &[".wa.Message"])

The plugin takes codec_strategy=table and repeatable codec_strategy_in=<path>=<strategy>. Rules match like preserve_unknown_fields_in: prefix, last match wins, rules over the global setting. A message with a oneof, map or group field, a custom string, bytes or collection type, MessageSet, or extension ranges with JSON stays unrolled, and so does every message that holds one. table_plan.rs computes that closure and reports it in one TableCodecFallbackSummary warning, silent when the user's own Unrolled rule is the cause. A rule that names such a message by its exact path is an error. compile() errors on rustc older than 1.77, and the MSRV job now also tests the table code on 1.77, so its timeout goes from 10 to 20 minutes.

The 1,644 KB to 817 KB figure in #463 was measured with oneofs and maps flattened, so a real schema saves less until the follow-up that lets table messages hold unrolled, extern and well-known-type children.

The conformance suite does not run under Table: TestAllTypesProto3 has oneofs and maps, so its messages fall back. Parity rests on buffa-test, which compiles each schema twice under renamed packages and compares bytes, sizes, decoded values and errors, including on truncated, bit-flipped and noise input. Under an experiment that set all 66 buffa-test protos to Table, 602 tests pass with 119 table messages.

A table message differs from an unrolled one in three ways, documented in the guide: a length past the end of its enclosing message fails at once with UnexpectedEof; merge_field cannot gather a non-contiguous buffer, so a type that another crate or run uses as a group or DELIMITED field must stay Unrolled; and clear() releases capacity. The generated code is tied to buffa::table, so regenerate it whenever buffa updates.

About 1,300 lines are outside test files, well over the 250-line guideline.

Followed by #475 (message fields that hold messages without a table), #476 (oneof) and #477 (map), stacked in that order.

[github-actions]

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