feat(blocks): add bounded email navigation (Issue #96 Slice 1)

[TheAggressive]

Summary

Implements the Navigation slice of #96. This PR does not close #96.

• Adds campaignbridge/navigation to the canonical grammar, editor allowlist and nesting rules, metadata registration, and deterministic compiler registry.
• Stores one to five explicit HTTPS links with labels limited to 24 plain-text characters. Malformed attributes, unsafe URLs, unsupported styles, and invalid nesting produce blocking diagnostics.
• Emits escaped links in an accessible presentation table, a mobile stacking rule, and one label and URL per plain-text line. It uses Brand Kit text color and body font.
• Adds golden HTML and text fixtures, compiler and contract tests, editor registration coverage, and a browser authoring test.
• Extends the client compatibility matrix merged in test(email): add client compatibility fixture matrix #103 to exercise Navigation across the universal, Outlook Word, Gmail, and Apple Mail profiles. Records the unverified Gmail body-style stacking behavior.

Validation

• pnpm qa:fast passed, including 19 JS suites and 133 tests.
• Full PHPUnit passed: 1,112 tests, 5,246 assertions.
• Focused navigation and client compatibility tests passed: 34 tests, 427 assertions.
• PHP style checks, JSON parsing, and diff whitespace checks passed.
• The prior branch CI passed, including browser E2E, build, and package verification. Fresh CI is running for this update.

Compatibility gate

Issue #72's fixture matrix is merged in #103. This branch now tests Navigation output against that matrix. The fixtures are structural regression evidence and do not claim visual rendering in real clients.