Harden directive/filter edge-case handling and add executable edge matrix coverage

[Copilot]

The engine still had edge-case failures in directive/filter parsing, especially around {{ ... }} expressions in attributes and filter chains containing literal pipes. This PR hardens lexer/compiler/filter behavior and adds a focused edge-case matrix to prevent regressions across directives and core filters.

• Lexer tag extraction robustness

  • Lexer::extractTag now tracks mustache depth while scanning tags.
  • Prevents premature tag termination when > appears inside {{ ... }} (e.g. array syntax with => in <Include data={{ ... }}>).

• Filter-chain parsing correctness

  • Compiler::compileExpression now uses a delimiter-aware splitter for |.
  • Pipes inside quoted strings/parenthesized args no longer split filters incorrectly (e.g. join(" | ")).

• String/array filter hardening

  • StringFilters: safer UTF-8-aware truncation helpers with non-mbstring fallbacks.
  • FilterRegistry::reverse: multibyte-safe string reverse when mb_* is available.
  • ArrayFilters::join: stable handling for non-scalar elements via deterministic stringification.
  • ArrayFilters::json: partial-output mode to avoid hard failures on non-encodable segments.

• Edge-case matrix for directives + filters

  • Added examples/edge_matrix.php with executable scenarios covering:
    • If/ElseIf/Else, Foreach loop metadata, Set, Include with expression data, Block
    • UTF-8 string filters, numeric/date filters, array filters, chained filters with argument edge cases

• Docs

  • README now includes the edge-matrix command in the quick validation section.

// Previously fragile: '|' inside filter args could be split as another filter
{{ value | join(" | ") }}

// Now parsed correctly as one filter call with one argument.