codegen: string_type knob for configurable string field types (#127)

[iainmcgin]

Summary

Implements #127 — the user-facing string_type codegen knob, letting proto string fields map to a small-string-optimized type instead of String.

Stacked on #143 (the runtime support). Review #143 first; this branch contains its commits plus the codegen + build API.

API

use buffa_build::StringRepr;
buffa_build::Config::new()
    .string_type(StringRepr::SmolStr)                              // broad default
    .string_type_in(StringRepr::CompactString, &[".my.pkg.Msg.body"]) // narrow override
    .files(&["proto/my_service.proto"])
    .includes(&["proto/"])
    .compile()?;

StringRepr is String (default), SmolStr, EcoString, or CompactString. Rules are ordered, last-match-wins (call the broad string_type first, then string_type_in overrides). The consumer enables the matching buffa feature (smol_str/ecow/compact_str). Mirrors the existing use_bytes_type machinery.

What changes in generated code

• Owned struct field type for singular / optional / repeated string fields and oneof string variants.
• Decode: String keeps the in-place merge_string fast path; other reprs use decode_string_to. clear() resets non-default reprs to Default::default() (the SSO types may be immutable). View→owned builds via From<&str>. Text format and proto2 [default = "..."] honor the repr. EcoString fields get an arbitrary shim.
• Unchanged: wire format, view types (still &str), and map<_, string> keys/values (always String). Encode/size paths need no change — the SSO types deref-coerce to &str.

Default String output is byte-for-byte identical — verified by regenerating the checked-in WKT and bootstrap descriptor types (the latter is proto2 with string fields, oneofs, defaults, and text format): zero diff.

Testing

cargo test --workspace and clippy --workspace --all-targets -D warnings pass. An end-to-end buffa-test module compiles string_types.proto with a SmolStr default + CompactString/EcoString overrides and covers binary / JSON / text / view round-trips, clear(), field-type pinning across all string shapes (incl. oneof payload), and a proto2 [default] fixture. Built with and without the arbitrary feature.

Note: there is pre-existing repo-wide cargo fmt drift in this environment (every committed file differs under the local rustfmt versions); a canonical cargo fmt pass may be wanted before merge.

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