feat: implement odin_test rule with shared compilation helper

[clappingmonkey]

Summary

Implements the odin_test Bazel rule for running Odin's built-in test framework (@(test)-annotated procedures) within Bazel's test infrastructure.

Design

• Uses odin build -build-mode:test to compile a test binary at build time (cacheable, hermetic)
• Bazel's test runner executes the binary at test time (exit 0 = pass, non-zero = fail)
• Follows the same pattern as rules_go (go_test) and rules_rust (rust_test)
• Injects ODIN_TEST_FANCY=false by default for CI-friendly output (no ANSI escape codes)
• Test runner config exposed via defines attr (compile-time constants in Odin)

Refactoring

Extracts compile_odin_binary() shared helper in common.bzl to eliminate code duplication between odin_binary and odin_test. Moves OdinLibraryInfo provider to common.bzl to resolve a circular dependency.

Changes

• odin/private/common.bzl — OdinLibraryInfo + compile_odin_binary() shared helper
• odin/private/binary.bzl — Refactored to use shared helper
• odin/private/library.bzl — Imports OdinLibraryInfo from common.bzl
• odin/private/test.bzl — New: odin_test rule implementation
• odin/defs.bzl — Re-exports odin_test and OdinLibraryInfo
• e2e/smoke/tests/smoke_test.odin — New: example test (3 @(test) procedures)
• e2e/smoke/BUILD.bazel — Adds odin_test target with library deps
• .github/workflows/check.yaml — bazel build → bazel test in smoke step
• README.md — Documents odin_test rule and test runner defines

Verification

• bazel build //... (ruleset root): PASSES
• bazel test //... (e2e/smoke): smoke_test PASSED in 0.7s
• bazel run //:hello (e2e/smoke): binary still works correctly
• pre-commit run --all-files: all checks pass
• @change-validator: APPROVED WITH CONCERNS (docs addressed, no blockers)

Checklist

• Conventional commit title
• Tests pass locally
• Pre-commit hooks pass
• README updated
• No breaking changes to existing public API