[core] Implement <esi:choose>, <esi:when>, <esi:otherwise> (#162)

[s2x]

Summary

Implements conditional content selection via <esi:choose>, <esi:when>, and <esi:otherwise> elements in the core ESI parser.

Closes #162

Changes

• mesi/parser.go: Added evaluateTest(), extractChooseBlocks(), processChooseBlock(), and depth-aware helper functions for parsing nested choose/when/otherwise blocks
• mesi/parser_test.go: 20+ unit tests covering all acceptance criteria
• docs/features.md: Updated feature matrix (⚠️ → ✅) and notes
• tests/fixtures/: Added e2e fixture (13-esi-choose)

Implementation details

• Boolean literal test expressions (true/false/0/1)
• $(...) variable resolution in test attributes
• Depth-aware block extraction (handles nested <esi:choose>)
• Supports nesting inside <esi:try> (both attempt and except blocks)
• Short-circuit evaluation (first matching when wins)
• Graceful degradation for malformed input

Test coverage

• test="true" → renders when body
• test="false" → renders otherwise
• All false, no otherwise → empty output
• First match wins (short-circuit)
• Nested include inside when is processed
• Empty test → false
• Malformed (missing close tags) → no panic
• $(VAR) resolution in test
• Multiple choose blocks in document
• Nested choose blocks
• Choose inside try/attempt and try/except