P30 adds a compiled tiny runtime

P30 keeps the P29 MMIO platform and changes the software workflow. The directed program now comes from runtime/start.S plus runtime/link.ld, built with riscv64-elf-gcc, converted to a byte image, and sent through the UART loader.

Commands:

make -C projects/30_rv32i_tiny_runtime/test
make -C projects/30_rv32i_tiny_runtime/test act4

Results:

check	result
Compiled tiny runtime	`PASS`
Runtime UART sequence `RTE`	`PASS`
Runtime timer interrupt	`PASS`
Runtime external interrupt	`PASS`
ACT4/Sail `rv32i/I`	`PASS=39 FAIL=0 NOT RUN=0`
Hardening	`NOT RUN`
Official privileged tests	`NOT RUN`

One subtle toolchain detail: modern assemblers require rv32i_zicsr for CSR instructions. The core already implements those CSR opcodes; the build flag is the assembler being precise, not a new full privileged-compliance claim.

The important shift is that future tests can now be programs with labels, sections, linker symbols, and disassembly, instead of piles of hand-encoded words in a testbench.

Next, P31 can either introduce a tiny C-level runtime on top of this assembly entry or start broadening the machine-mode software tests.