riscv64: real trap entry with save/restore and SRET return

Punch list §25 item 0.2 complete.

Verified: riscv64 builds clean and boots to the ok> prompt with no regression;
dict_hash 0x3d4e1daf289da94f, unchanged from item 0.1's baseline. Disassembly
confirms the 320-byte frame, all 16 integer caller-saved registers, the FS
check, and SRET on exit; riscv64_trap_entry lands at 0x414fa8, 4-byte aligned
as stvec direct mode requires.

Not verified, and the item says so: neither new path was exercised. No timer is
armed until 0.3, so riscv64_interrupt_handler never ran, and no exception
occurred during boot, so the fatal path was not observed -- it is preserved
structurally, same branch to the same unchanged handler. This is why C2
rewrote the acceptance to no-regression rather than to having taken and
returned from a trap.

Register set is the LP64D psABI caller-saved list, not this document's summary:
integer ra/t0-t6/a0-a7 (16), FP ft0-ft11/fa0-fa7 (20) plus fcsr, and sepc +
sstatus. Callee-saved registers are the C handler's responsibility.

The FP half is conditional on sstatus.FS != Off, which the item did not
anticipate. Nothing in boot.S or kernel_entry.S programs FS, so its value is
whatever firmware leaves; touching an f-register with FS == Off raises an
illegal-instruction trap, and doing that inside the trap handler would be
unrecoverable. Omitting the FP save is not an option either -- the built
riscv64 image contains 530 FP instructions (fld, fmul.d, fcvt.lu.d among them),
confirming B2's finding against the binary rather than the build flags alone.
So the save is conditional, and sstatus is restored after the f-registers.

Dispatch: scause bit 63 routes to riscv64_interrupt_handler with scause in a0;
cause 5 (supervisor timer) calls heartbeat_tick(). Other causes are ignored
rather than fatal -- none are enabled to arrive. Everything else still falls
through to riscv64_exception_handler, unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

experiments/bare_metal/runs/doe-riscv64-20260803-125133.csv

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