feat: implement KEYS command

[ithustle]

Summary

• Add KEYS command (CMD 0x0D) that lists all non-expired keys in numbered format (1) key1\n2) key2\n)
• Dedicated send_keys_reply() tags the response frame with CMD_KEYS so the client correctly handles empty lists ((empty list)) vs key listings (raw text, no quotes)
• Server handler iterates hash table with safe next pointer caching, lazy expiration via check_and_expire(), and dynamic buffer with realloc growth (max ~65500 bytes)
• 4 integration tests: empty store, multiple keys, expired key exclusion, key after DEL

Files changed (11)

• command_defs.h — #define CMD_KEYS 0x0D
• command_parser.h/c — construct_keys_command()
• command_registry.h/c — send_keys_reply()
• server_command_handlers.h/c — handle_keys_command() + registration
• client_command_handlers.h/c — cmd_keys(), command_table[], cmd_unknown() filter, CMD_KEYS response branch
• README.md — KEYS added to Server commands table
• test_integration.c — 4 new tests

Test plan

• make -f Makefile.fkvs build — compiles without new warnings
• ctest --output-on-failure — all 3 test suites pass (39 tests total)

Test project /Users/ithustle/dev/server/fkvs
Start 1: CounterTest
1/3 Test #1: CounterTest ...................... Passed 0.00 sec
Start 2: StringUtilsTest
2/3 Test #2: StringUtilsTest .................. Passed 0.00 sec
Start 3: IntegrationTest
3/3 Test #3: IntegrationTest .................. Passed 18.77 sec

100% tests passed, 0 tests failed out of 3

Total Test time (real) = 18.77 sec