feat(decode): implement optimized dual-path parser with enhanced error reporting per #10

[harningt]

Description

Overview

This PR implements positioned error reporting with line and column numbers.
It provides human-readable diagnostic messages for common syntax and validation
errors (e.g., trailing commas, duplicate/missing values, and missing separators) without introducing any performance overhead on successful parsing paths.

Key Improvements

1. Dual-Path Parsing Architecture:
   • Fast-Path: Normal decodes execute using the original fast parser without capturing token positions or incurring pcall loop overhead.
   • Diagnostic-Path: If the fast-path parser fails, the decoder catches the error and executes a secondary diagnostics parser that captures position indices (lpeg.Cp()), wrapping the token state machine in a pcall to format precise line and column location info.
2. Context-Aware Error Messages:
   • Upgraded state-machine assertions to provide clear, actionable explanations (e.g. "Expected value after ':'", "Expected key before ':'", and trailing comma disallowance reasons).
   • Captured error as a local upvalue to maintain strict compliance with sandbox environments (_ENV = nil).
3. 1-Indexed Line & Column Info:
   • Fixed a bug in get_invalid_character_info where line numbers were 0-indexed, and resolved a regex limitation that prevented matching single-character lines.
4. Behavior Clarifications & Code Annotations:
   • Added comments and README sections clarifying the interaction between trailingComma and allowEmptyElement (wherein a trailing comma is treated as a trailing empty element if allowEmptyElement is enabled).

Performance Comparison

Using the baseline test matrix benchmark (tests/dataTest.lua decoding a 500-key object 1,000 times on Lua 5.4):

• Baseline (develop): 0.470s
• Naive position tracking (Always-on): 0.666s (~41% overhead)
• Dual-Path Design (This PR): 0.481s (~2% overhead / within run-to-run noise)

Testing & Validation

• Integrated a new unit test suite tests/lunit-error-reporting.lua asserting exact line/column positions across 15 failure conditions.
• Ran tests against the full matrix (Lua 5.1–5.5, LuaJIT, MoonJIT) with 100% pass rates.

[codecov]

Codecov Report

❌ Patch coverage is 89.06250% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.42%. Comparing base (d8f39af) to head (e883555).
⚠️ Report is 7 commits behind head on develop.

Files with missing lines	Patch %	Lines
lua/json/decode/util.lua	37.50%	5 Missing ⚠️
lua/json/decode.lua	94.11%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #54      +/-   ##
===========================================
- Coverage    95.90%   95.42%   -0.49%     
===========================================
  Files           18       18              
  Lines         1026     1071      +45     
===========================================
+ Hits           984     1022      +38     
- Misses          42       49       +7     

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