feat: MEMVAR system — PUBLIC/PRIVATE dynamic variables

Complete Harbour-compatible MEMVAR implementation:
- PUBLIC: global scope, persist until program end
- PRIVATE: function scope + called functions, auto-release on return
- Shadowing: PRIVATE can shadow PUBLIC, restored on scope exit
- Nested: multi-level PRIVATE scoping with save/restore stack
- Thread.PushMemvar/PopMemvar: stack-based memvar access
- Thread.DeclarePublic/DeclarePrivate: declaration helpers
- MacroEval: &cVar now looks up memvars (was returning string)
- Shutdown: Phase 4 clears all memvars on all threads
- Case-insensitive: all lookups uppercased

Tests: 12 tests including:
  PUBLIC create/update, case-insensitive, PRIVATE basic,
  shadow/restore, nested 3-level shadow, new var cleanup,
  release, releaseAll, names, thread integration, macro access

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/.pdca-status.json

@@ -1,6 +1,6 @@
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@@ -33,9 +33,9 @@
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       },
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@@ -5898,6 +5898,60 @@
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 }