feat: integrate pl.strict testing and add to CI matrix per #35

[harningt]

Description

This Pull Request integrates support for testing luajson under Penlight's strict variable check module (pl.strict) and adds it to the CI test matrix.

To make pl.strict compatible with the test suite (avoiding global lookup errors on optional globals in luacov, luarocks, and lunit), this change:

1. Defers Strict Activation: Postpones enabling strict mode until after core frameworks (luarocks, luacov, etc.) have loaded.
2. Pre-declares Environment Globals: If TEST_STRICT=pl.strict is used, the hook pre-declares expected nil-check globals accessed by luacov, luarocks, and lunit.
3. Guards Nil Index: Overrides _G's __index to handle and skip nil key lookups, preventing formatting crashes within pl.strict.
4. CI Matrix Integration: Configured the lua 5.3 matrix item to run with strict_module: "pl.strict".
5. On-demand Penlight Setup: Updated run_tests.sh to install penlight via luarocks only when TEST_STRICT=pl.strict is specified.

---

Verification

• Ran regression and unit tests under TEST_STRICT=pl.strict locally on Lua 5.3:

  TEST_STRICT=pl.strict ./run_tests.sh "lua 5.3" "1.0.1-1"

• Ran full local matrix to verify no regressions:

  TEST_STRICT=1 make test-matrix

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.90%. Comparing base (30bac5e) to head (d8f39af).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #53   +/-   ##
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  Coverage    95.90%   95.90%           
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  Files           18       18           
  Lines         1026     1026           
========================================
  Hits           984      984           
  Misses          42       42           

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