📦🔧：keep email validation linear and precise

[DerekNonGeneric]

The email guard's backtracking pattern took quadratic time on invalid dotted
domains and could block the event loop. Its value is string return type also
made TypeScript exclude every string after a failed syntax check.

This replaces the pattern with linear scans while preserving its accepted
syntax, introduces a branded Email refinement, and adds compatibility,
narrowing, whitespace, and adversarial-input regressions.

This is a security-sensitive validation fix, so no public vulnerability issue
was opened.

Summary by CodeRabbit

• Bug Fixes
  • Email validation now handles long invalid addresses in linear time and rejects whitespace, repeated or missing @ symbols, and domains with empty sections around a dot.
• Developer Experience
  • Successful validation now narrows values to the exported Email type.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Essentials

Run ID: d8928d46-52cb-45b6-87a1-bf67893c75d2

📥 Commits

Reviewing files that changed from the base of the PR and between 8177c45 and 1c85392.

📒 Files selected for processing (3)

• .changeset/linear-email-validation.md
• packages/util-string/src/guards/is-email.ts
• packages/util-string/test/guards/is-email.ts

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📝 Walkthrough

Walkthrough

isEmail replaces its regular-expression check with explicit syntax checks and returns the exported tagged Email type for accepted values. Tests cover syntax, whitespace, long invalid input, and type narrowing. A major changeset records the update.

Changes

Email validation

Layer / File(s)	Summary
Validator and exported type packages/util-string/src/guards/is-email.ts, .changeset/linear-email-validation.md	isEmail uses explicit checks to reject non-strings, whitespace, malformed @ usage, and domains without nonempty segments around a dot. Its type predicate now narrows to the exported tagged Email type. The major changeset describes these changes.
Validation and type tests packages/util-string/test/guards/is-email.ts	Tests cover accepted and rejected syntax, whitespace characters, a long invalid address with a two-second expectation, and type narrowing. The test imports isEmail and Email from the package index.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 1c853

The validation change is mergeable after normal checks; no concrete compatibility regression was found.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the main change: linear and precise email validation. It is concise and specific, although the emojis are unnecessary.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files. (1 skipped: 1 unsupported.)

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[openinf-commit-queue]

The commit queue did not land this. See https://github.com/OpenINF/sdk/actions/runs/36962272189 — the label has been taken back off, so re-applying it is a deliberate second try.