fpga: rename "quirk" to "caveat"

"quirk" was unclear jargon; "caveat" matches the wording already used in
the README/CLAUDE.md ("Xilinx caveats"). Renames the struct field, the
FPGA_QUIRK_* macro, the fpga_info output and the docs. No behaviour
change.

modules/fpga/fpga.c

@@ -21,7 +21,7 @@ static const fpga_target fpga_registry[] = {
         .ir_jshutdown     = 0x0D,
         .ir_isc_disable   = 0x16,
         .proxy_bitstream  = NULL,   /* TODO Phase 2.5: bscan_spi_xcku15p.bit */
-        .quirks           = FPGA_QUIRK_CCLK_VIA_STARTUP,
+        .caveats          = FPGA_CAVEAT_CCLK_VIA_STARTUP,
     },
     /* Xilinx Kintex UltraScale XCKU040 (KCU105 eval board)
      * IDCODE_REGISTER and INSTRUCTION_OPCODE values come from
@@ -41,7 +41,7 @@ static const fpga_target fpga_registry[] = {
         .ir_jshutdown     = 0x0D,
         .ir_isc_disable   = 0x16,
         .proxy_bitstream  = "bscan_spi_xcku040.bit",
-        .quirks           = FPGA_QUIRK_CCLK_VIA_STARTUP,
+        .caveats          = FPGA_CAVEAT_CCLK_VIA_STARTUP,
     },
 };