🚀 feat: Multiple improvements for Nyash parser and LLVM backend

Parser improvements:
- Added expression statement fallback in parse_statement() for flexible syntax
- Fixed ternary operator to use PeekExpr instead of If AST (better lowering)
- Added peek_token() check to avoid ?/?: operator conflicts

LLVM Python improvements:
- Added optional ESC_JSON_FIX environment flag for string concatenation
- Improved PHI generation with better default handling
- Enhanced substring tracking for esc_json pattern

Documentation updates:
- Updated language guide with peek expression examples
- Added box theory diagrams to Phase 15 planning
- Clarified peek vs when syntax differences

These changes enable cleaner parser implementation for self-hosting,
especially for handling digit conversion with peek expressions instead
of 19-line if-else chains.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/reference/language/LANGUAGE_REFERENCE_2025.md

@@ -0,0 +1,6 @@
+# Language Reference (2025)
+
+The canonical 2025 language reference currently lives here:
+- private/papers/reference/language/LANGUAGE_REFERENCE_2025.md
+
+This stub exists to provide a stable public path under `docs/reference/language/`.

docs/reference/language/README.md

@@ -0,0 +1,20 @@
+# Nyash Language Reference – Index
+
+This is the entry point for Nyash language documentation.
+
+- Full Language Reference (2025): reference/language/LANGUAGE_REFERENCE_2025.md
+- Syntax Cheat Sheet: quick-reference/syntax-cheatsheet.md
+- Phase 12.7 Grammar Specs (peek, ternary, sugar):
+  - Overview: development/roadmap/phases/phase-12.7/grammar-specs/README.md
+  - Token/Grammar: development/roadmap/phases/phase-12.7/ancp-specs/ANCP-Token-Specification-v1.md
+- Sugar Transformations (?., ??, |> and friends): parser/sugar.rs (source) and tools/nyfmt/NYFMT_POC_ROADMAP.md
+- Peek Expression Design/Usage: covered in the Language Reference and Phase 12.7 specs above
+
+Related implementation notes
+- Tokenizer: src/tokenizer.rs
+- Parser (expressions/statements): src/parser/expressions.rs, src/parser/statements.rs
+- MIR Lowering (expressions): src/mir/builder/exprs.rs and friends
+
+Navigation tips
+- The “reference/language/LANGUAGE_REFERENCE_2025.md” is the canonical long‑form reference; use the Cheat Sheet for quick syntax lookup.
+- Phase 12.7 files capture the finalized sugar and new constructs (peek, ternary, null‑safe).