cc: elide push/pop in kernel_outb/outw when port is a literal

[bboe]

Summary

The general non-const-value path in builtin_kernel_outb / builtin_kernel_outw was emitting an unconditional save-around-eval guard:

eval value → AX
push AX
eval port → DX        ; might clobber AX in the general case
pop AX
out dx, al

But when port is an Int literal, the port lowering is a single mov dx, <imm> that doesn't touch AX — so the save is dead. Adding an isinstance(port_arg, Int) branch elides the push/pop in that case.

Almost every kernel kernel_outb / kernel_outw call site uses a hex-literal port (0x21, 0x3F6, 0x300, ...), so this fires kernel-wide.

Impact

Kernel kasm: -1,980 bytes (on top of the -217 from PR #446). Folds fire in ata, fdc, ne2k, ps2, sb16, opl3, console, vga, serial, rtc.

Test plan

• Two new unit tests: test_kernel_outb_constant_port_runtime_value_no_push_pop, test_kernel_outw_constant_port_runtime_value_no_push_pop
• Pre-existing test_kernel_outb_variable_value_uses_push_pop (with a non-Int port) still exercises the general save-around-eval path
• Full unit suite: 458 passed
• Full matrix: test_asm (42), test_programs bbfs (89), test_programs ext2 (119), test_bboefs (6), test_cc_bits (110), test_cc_bitfields (12), test_cc_casts (6), test_cc_local_structs (10), test_cc_compatibility (57), test_kernel_archive (12), test_archive (12), test_pipeline_basic, test_floppy_boot, test_draw

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