Fix generator escaping / name-collision compile-break class

[aaylward]

Closes #43 (top-tier from the whole-project review). Four generator paths emitted C++ that silently fails to compile for legal-but-unusual Smithy models, with no generation-time diagnostic.

Fixes

• Enum value-set validation message interpolated raw wire values into a string literal (ValidationGenerator.writeEnumCheck); a value containing " or \ broke it. Now escaped in place via a new CppLiterals.escapeStringBody — byte-identical output for the common safe case, so no golden churn.
• @pattern raw-string delimiter: the pattern was emitted verbatim inside R"__smithy(...)__smithy", so a value containing the closing sequence )__smithy" terminated the literal early. Rejected at generation time with a message naming the shape and the fix.
• int64 minimum literals: @default and @range bounds emitted -9223372036854775808, which C++ can't parse (negation of a value one past int64_t max). Now emitted via the header-free INT64_MIN idiom (CppLiterals.int64Literal); only the minimum changes, every other magnitude stays byte-identical.
• Enum/union name-folding collisions: enum-constant and union-factory naming lower-case / strip separators / PascalCase the member name, so two distinct members (or a member named unknown) could fold to one C++ name → duplicate enumerator/method. Now detected and failed at generation time with a diagnostic naming both members (or the reserved kUnknown).

Testing

• New codegen unit tests, one per fix: enum-value escaping (asserts the escaped bytes appear and the raw form does not), raw-string-delimiter rejection (using a valid regex that contains the sequence — Smithy validates @pattern as a real regex), enum name-fold collision (foo_bar vs foo__bar, both fold to kFooBar — reachable because Smithy shape IDs are case-insensitive so case-only collisions can't occur), the kUnknown-sentinel collision, and the int64-min @range/@default idiom.
• gradle build spotlessCheck green; regeneration produces zero golden diff (git diff --stat -- examples protocol-tests is empty — the guards only fire on previously-broken input).
• bazel test //protocol-tests/... //examples/cafe/... (the enum/validation paths): 20/20 pass.

Checklist

• Tests added/updated for the change (five codegen unit tests covering each fix)
• bazel test //... and (cd codegen && gradle build spotlessCheck) pass locally (Beast/benchmark excluded per docs)
• Formatting clean (spotless; no generated-code churn)
• Architectural decisions recorded as an ADR (not applicable — bug fixes within existing conventions)

Note: a compile-the-generated-output test harness — which would catch this whole class structurally rather than case-by-case — is tracked separately in #48.

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https://claude.ai/code/session_01SyQAo21Pv6GYhHrkbQj8xQ

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