feat: add lab-rv32i-freertos-isr-drivers card

README.md

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+# K15 — Explicit FromISR, UART and GPIO wrappers
+
+K15 routes a real Hazard3 machine-external interrupt through the official
+FreeRTOS RISC-V trap path. The C++ layer keeps task and ISR views explicit:
+UartTaskView/GpioTaskView contain configuration and output operations, while
+UartIsrView/GpioIsrView expose only bounded register reads and clears.
+
+One IsrContext accumulates wake requests from two primitives:
+
+    UART RX bytes: 0x41, 0x42
+    static queue capacity: 1
+    accepted: 0x41
+    overflow policy: drop-newest -> 0x42 dropped
+    GPIO edge: task notification
+    higherPriorityTaskWoken: true
+    yield_if_needed calls: exactly 1
+
+The high-priority receiver consumes byte 0x41 and notification count 1, then
+drives GPIO output high. It runs before the interrupted stimulus continues,
+which proves the ISR-requested context switch.
+
+## Build
+
+    make check
+
+The gate includes a host contract test, freestanding ABI/storage checks and a
+real external-IRQ simulation on Hazard3.
+
+## Debug
+
+    riscv64-unknown-elf-gdb build/task01_isr_drivers/prog.elf
+    b isr_drivers_debug_checkpoint
+    p/x g_mcause
+    p g_rx_queued
+    p g_rx_dropped
+    p/x g_receiver_byte
+    p/x g_dropped_byte
+    p g_higher_priority_task_woken
+    p g_isr_yield_calls
+    p g_stimulus_after_seen_by_receiver
+    p g_receiver_done_at_stimulus_resume
+    p g_isr_drivers_pass
+
+## Scope boundary
+
+ISR code cannot call a blocking task API. Parsing, debounce and GPIO policy
+belong to a task. Backpressure is explicit: this exercise uses drop-newest and
+counts the dropped byte. The handler creates one IsrContext and calls
+yield_if_needed once, after all FromISR publications and peripheral clears.