Warn when IDL identifiers don't follow naming conventions

Summary

weaveffi lint surfaces the advisory warnings collected in crates/weaveffi-core/src/validate/warnings.rs, but none of them check identifier casing. The naming conventions in docs/src/reference/naming.md (snake_case for functions and parameters, PascalCase for structs, enums, and interfaces) are currently unenforced, so a stray createItem or kv_error sails through silently even though the per-target generators assume conventional input casing when they re-case identifiers.

Proposed change

Add new ValidationWarning variants to crates/weaveffi-core/src/validate/warnings.rs, for example:

• NonSnakeCaseFunction { module, function } for function and parameter names that aren't snake_case
• NonPascalCaseType { module, type_name } for struct, enum, interface, and error domain names that aren't PascalCase

Then extend collect_warnings to walk the API and emit them, and add Display messages that tell the user the expected casing (see the existing variants for the message style).

Acceptance criteria

• weaveffi lint warns on a function named createItem and a struct named kv_item, with messages that name the offending identifier and the expected convention
• Conventionally cased IDLs (e.g. everything under samples/) produce no new warnings
• Unit tests in crates/weaveffi-core/src/validate/tests.rs cover both the warning and the no-warning cases, following the existing warning_* test pattern
• New public items carry doc comments (the crate uses #![deny(missing_docs)])

Pointers

• Warning enum and collector: crates/weaveffi-core/src/validate/warnings.rs
• Existing tests to mimic: warning_large_enum_variant_count and friends in crates/weaveffi-core/src/validate/tests.rs
• CLI surface: cmd_lint in crates/weaveffi-cli/src/commands/validate.rs
• Conventions reference: docs/src/reference/naming.md

Suggested commit message

feat: warn when IDL identifiers don't follow naming conventions

Start with the warning variants and collector in crates/weaveffi-core/src/validate/warnings.rs, then read warning_large_enum_variant_count and related tests in crates/weaveffi-core/src/validate/tests.rs. Check the naming conventions in docs/src/reference/naming.md and the lint entry point in crates/weaveffi-cli/src/commands/validate.rs. Done means the specified casing warnings and messages work, conventional samples stay quiet, tests cover both cases, and new public items have doc comments.