fix: harden IRCv3 tag parsing and multiline protocol helpers

[louzt]

Summary

This PR is a protocol-correctness hardening pass for generic IRCv3 helpers.

It is intentionally separate from the browser-native translation PR because these changes improve the shared IRC substrate used by multiline handling, reply/react/typing tags, and future tag-driven features regardless of translation.

What this changes

• parses IRCv3 message-tag values with proper escaping semantics
• preserves everything after the first = in message-tag and ISUPPORT values
• decodes \\x20 in ISUPPORT values without truncating richer token payloads
• preserves whitespace required for correct draft/multiline-concat behavior
• generates batch ids using spec-compliant characters only
• splits multiline payloads by UTF-8 byte length instead of JavaScript code-unit length
• replaces deprecated substr() usage with slice() while touching the same helper path
• adds focused regression tests for tags, ISUPPORT parsing, batch ids, whitespace preservation, and multibyte payload splitting

Why keep this separate

These are lower-level protocol helpers, not translation-specific product changes.
Reviewing them independently keeps the translation PR focused while making the protocol hardening reusable for the rest of the client.

Standards references

• IRCv3 Message Tags: https://ircv3.net/specs/extensions/message-tags
• IRCv3 Multiline: https://ircv3.net/specs/extensions/multiline
• IRCv3 Batch: https://ircv3.net/specs/extensions/batch

What this does not change

• no new user-facing feature flags
• no new UI surface
• no change in product scope; this is correctness / interoperability hardening only

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 58b86179-0999-4db0-8fe4-3be1faa74207

📥 Commits

Reviewing files that changed from the base of the PR and between e7d7899 and 0df146a.

📒 Files selected for processing (4)

• src/lib/ircUtils.tsx
• src/lib/messageProtocol.ts
• tests/lib/ircUtils.test.ts
• tests/lib/messageProtocol.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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