refactor: rename llcppg.symb.json to llcppg.sym.json

[fennoai]

Summary

Renames the symbol info file from llcppg.symb.json to llcppg.sym.json to align with industry-standard abbreviation conventions.

• Uses sym (standard in compilers/linkers like nm, objdump, LLVM) instead of symb
• Improves consistency and readability
• Reduces naming confusion for new contributors

Changes

• ✅ Renamed 20 test data files from llcppg.symb.json to llcppg.sym.json
• ✅ Updated constant LLCPPG_SYMB in config/config.go
• ✅ Updated all code references in 6 source files
• ✅ Updated documentation in CLAUDE.md and doc/en/dev/llcppg.md
• ✅ All unit tests passing
• ✅ Build successful
• ✅ Code formatted and vetted

Test Plan

• Build project: go build -v ./...
• Run unit tests: go test -v ./config ./internal/name ./internal/arg ./internal/unmarshal
• Run go fmt and go vet
• Verified all references updated

Closes#579

🤖 Generated with codeagent

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.19%. Comparing base (7955d98) to head (0a2c844).

Files with missing lines	Patch %	Lines
cmd/llcppg/llcppg.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #580 +/- ##
=======================================
Coverage 84.19% 84.19% =======================================
Files 27 27 Lines 2746 2746 =======================================
Hits 2312 2312 Misses 391 391 Partials 43 43 

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