Resolve #2746: Validate explicit Cron distributed owner IDs

[ayden94]

Summary

Cron normalized user-provided distributed ownerId without validation, so invalid or empty owner IDs could enter Redis lock ownership state despite the distributed-lock contract. This PR validates explicit distributed.ownerId at the module configuration boundary before scheduler or Redis lifecycle setup.

Linked context: #2746

Changes

• packages/cron/src/module.ts: Add normalizeDistributedOwnerId(...) that trims an explicit ownerId and rejects blank, empty, or non-string values during module option normalization, before scheduler or Redis lifecycle setup. Mirrors the existing normalizeRedisClientName(...) and assertValidDistributedLockTtlMs(...) boundary-validation pattern.
• packages/cron/src/module.test.ts: Add regression tests covering blank, empty, and non-string explicit ownerId rejection during normalization, before Redis probe I/O, and during bootstrap; plus trim and valid-ownerId preservation tests.
• packages/cron/README.md and packages/cron/README.ko.md: Document that fluo trims and validates explicit distributed.ownerId before scheduler or Redis lifecycle setup so invalid or empty owner identifiers cannot enter Redis lock ownership state.
• .changeset/cron-validate-distributed-owner-ids.md: Patch changeset for @fluojs/cron because README documents distributed.ownerId and this is a behavior change at the module configuration boundary.

Testing

• pnpm vitest run packages/cron — 92 tests passed (4 test files), including the new ownerId validation regression tests.
• pnpm build — succeeded.
• pnpm typecheck — succeeded.
• pnpm lint — exited 0; no new diagnostics in changed files.

Release impact

• This PR has consumer-visible release impact and includes a changeset.

Public export documentation

• Changed public exports include a source-level summary. (normalizeCronModuleOptions already documented; no new public export added.)
• Changed exported functions document matching @param / @returns tags where applicable.
• Source @example blocks and README scenario examples still play complementary roles.

Behavioral contract

• No documented behavioral contracts were removed without migration notes.
• New behavioral contracts are documented in the affected package README.
• Intentional limitations are explicitly stated (not silently removed).
• Runtime invariants are covered by regression tests.

Platform consistency governance (SSOT)

• SSOT English/Korean mirror structure remains synchronized for changed governance docs. (README.md and README.ko.md both updated.)
• If platform contract docs changed, companion updates include discoverability/docs index, tooling or CI enforcement, and regression-test evidence. — Not applicable; no platform contract docs changed.
• Any package README alignment/conformance claims are backed by the applicable platform harness tests. — Not applicable; no platform harness claims changed.

Closes #2746