codegen: idiomatic UpperCamelCase enum aliases

[iainmcgin]

Summary

Closes #13. Protobuf enum values are SHOUTY_SNAKE_CASE (e.g. RULE_LEVEL_HIGH), which is unidiomatic in Rust. This makes codegen additionally emit idiomatic UpperCamelCase aliases.

The proto names remain the definitive variants — this is purely additive:

pub enum RuleLevel { RULE_LEVEL_UNKNOWN = 0, RULE_LEVEL_HIGH = 1, /* ... */ }

impl RuleLevel {
    pub const Unknown: Self = Self::RULE_LEVEL_UNKNOWN;
    pub const High: Self = Self::RULE_LEVEL_HIGH;
}

So RuleLevel::RULE_LEVEL_HIGH keeps working unchanged (including Debug output), and RuleLevel::High is now also available. The aliases are associated consts, which work in pattern position with exhaustiveness checking preserved (verified on the 1.85 MSRV); a non-exhaustive match error just names the underlying SHOUTY_SNAKE_CASE variant.

Collision handling

The CamelCase conversion is lossy (FOO_BAR and FOO__BAR both → FooBar). The rule is all-or-nothing per enum: if any two values collide after conversion — or a value would yield an invalid identifier — no aliases are emitted for that enum. A structured CodeGenWarning (surfaced as a cargo:warning= by buffa-build, on stderr by the generators) and a doc note on the generated enum explain why, naming the exact values. This guarantees a match is never forced to mix conventions.

Edge cases handled and tested: lossy fold, the silent variant↔const shadow (FOO_BAR vs a literal FooBar), empty/leading-digit remainders after prefix stripping (fall back to unstripped), Rust keywords (KIND_SELF → Self_), and already-CamelCase values (skipped as redundant). Prefix stripping is all-or-nothing per enum, so the result never mixes stripped and unstripped names.

API

• New CodeGenConfig::idiomatic_enum_aliases (default on — additive, so backward-compatible) and a buffa_build::Builder::idiomatic_enum_aliases setter.
• New generate_with_diagnostics returning Vec<CodeGenWarning>; the existing generate() delegates and discards them, so connectrpc-codegen is unaffected.
• Regenerates the checked-in WKT, bootstrap descriptor, and logging-example code.

Reviewed by rust-code-reviewer and rust-api-ergonomics-reviewer (two rounds); all Critical/High/Medium findings addressed.

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