[core] Implement <esi:inline> element (#161)

[s2x]

Description

Implements the <esi:inline> element per the W3C ESI 1.0 specification. Content inside <esi:inline> is now rendered verbatim without further ESI processing — serving as an escape hatch for literal ESI markup in output.

Closes #161

Changes

• mesi/inline.go (new) — processInlineBlock() extracts body content using quote-aware tag-end detection
• mesi/parser.go — Added ESI_INLINE handling in MESIParse() and processAttemptContent() (same approach as ESI_VARS/ESI_TRY/ESI_CHOOSE)
• mesi/parser_test.go — 8 test functions with 18 test cases covering:
  • Basic verbatim rendering
  • ESI markup escape hatch (include inside inline not processed)
  • HTML markup passthrough
  • Empty inline body
  • Inline with surrounding content
  • Attributes on inline tag
  • Inline inside esi:attempt and esi:except
  • Quote-aware tag extraction edge cases
  • Multi-byte UTF-8 preservation
• tests/fixtures/ — 3 e2e test fixtures
• docs/features.md — Updated feature matrix from ⚠️ to ✅

Verification

• All existing tests pass
• New unit tests pass
• Code reviewed separately