add arena allocator for string allocations

[cs01]

Summary

• Replaces GC_malloc_atomic with bump-pointer arena allocation (cs_arena_alloc) for all temporary string allocations across codegen
• Arena blocks are backed by Boehm GC (GC_malloc for metadata, GC_malloc_atomic for data), so no memory leaks — GC still manages block lifetime
• 1MB arena blocks with 8-byte aligned bump pointer — faster than individual GC_malloc_atomic calls for string-heavy workloads

What changed

• New c_bridges/arena-bridge.c — bump-pointer arena allocator (50 lines)
• Converted ~45 GC_malloc_atomic → cs_arena_alloc calls across: string concat, substr, trim, repeat, pad, replace, case conversion, charAt, split, number-to-string, path ops, date formatting, JSON stringify, process.cwd, os.hostname, String.fromCharCode, regex match extraction, for-of string iteration
• Non-string allocations (array data buffers, Uint8Array, crypto, file I/O, class instances) remain on Boehm GC
• Build system: arena-bridge added to build-vendor.sh, compiler linkers, CI, and cross-compile SDK

Test plan

• npm run verify:quick passes (742 tests + self-hosting stage 0+1)

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