[core] Implement <esi:vars> and $\(...\) expression evaluation

[s2x]

Summary

Implements <esi:vars> block parsing and $(...) expression evaluation for ESI variable substitution.

Closes #164

Changes

mesi/config.go

• Added Variables map[string]string — holds ESI variable definitions from <esi:vars> blocks and pre-populated values
• Added RequestHeaders http.Header — for $(HTTP_HEADER{Name}) resolution
• Added RequestCookies map[string]string — for $(HTTP_COOKIE{name}) resolution
• Added RequestQuery map[string]string — for $(QUERY_STRING{param}) resolution

mesi/vars.go (new)

• evaluateExpression(expr, config) — resolves $(...) patterns:
  • $(NAME) → looks up in config.Variables (explicit variables take precedence)
  • $(HTTP_HEADER{Name}) → resolves from config.RequestHeaders
  • $(HTTP_COOKIE{name}) → resolves from config.RequestCookies
  • $(QUERY_STRING{param}) → resolves from config.RequestQuery
  • Undefined → empty string
• parseVarsBlock(rawContent) — extracts <esi:variable name="..." value="..."/> definitions from <esi:vars> blocks

mesi/parser.go

• Process vars tokens inline: parse variables, merge into config, emit empty output
• Apply evaluateExpression to static text content
• Apply evaluateExpression to include src and alt attributes before fetching

Tests

• TestEvaluateExpression (15 cases): variable substitution, HTTP_HEADER, HTTP_COOKIE, QUERY_STRING, precedence
• TestParseVarsBlock (7 cases): parsing variable definitions
• TestMESIParseWithVarsAndTextSubstitution (6 cases): end-to-end text substitution
• TestMESIParseWithVarsAndInclude: include src resolution
• TestMESIParseWithVarsBlockAndInclude: vars block + include
• TestMESIParseWithVarsIncludeUsingBACKEND: full URL variable
• TestMESIParseWithVarsAltAttribute: alt attribute resolution
• TestMESIParseVarsAndTemplatedURL: multiple $(...) in one URL
• E2E fixture 11-esi-vars.html: text substitution via <esi:vars> block

Documentation

• Updated docs/features.md: <esi:vars> / $(...) marked as ✅ (was ⚠️)
• Added note explaining variable substitution support