Spec M5 — `#section`, `#opcode`, `emit()`, `reloc()`, `sys.mc` via `svc`

Prerequisite: M4. This is the milestone that proves "emit object-file code and say where it goes".

Parser / directives #

#section SEG SECT FLAGS[ ALIGN]: creates (or selects) the section via sec_new and makes it the current section for functions and globals declared after it, until the next #section. #section with no arguments reverts to the default. Flags is a folded constant (e.g. 0x80000400).
#opcode name(p1, p2, ...) EXPR: registers an encoder in a linear table {name, params, tmpl}. A call name(args) in a function body: each arg must fold to a constant (otherwise error "non-constant #opcode argument"); the template is folded with the parameters substituted and the resulting 32-bit word goes into the instruction buffer as I_RAW. Not a symbol.
emit(CONST): same effect, a raw word. reloc(TYPE, "symbol"): registers a relocation for the next word emitted in the function; TYPE ∈ BRANCH26 PAGE21 PAGEOFF12 UNSIGNED (predefined constants, internal #defines). An unknown symbol becomes an undefined external (sym_ref).
The encoder table and the current section are flat data, in insertion order.

I_RAW in the buffer carries the word; --dump-asm prints .word 0x.........
The pending relocation from reloc() is attached to the offset of the next I_RAW/instruction.
Uphold the plan's promise: the prologue writes args to the frame without touching x0..x7, and the return value of a function that ends with svc is the x0 left by the syscall (the epilogue does not touch x0 when there is no return).

lib/sys_svc.mc: the same interface as lib/sys.mc (open/read/write/close/exit) implemented with #opcode mov16(rd, imm) 0xD2800000 | (imm << 5) | rd, #opcode svc(imm) 0xD4000001 | (imm << 5) and BSD numbers exit=1 read=3 write=4 open=5 close=6.

030-section.mc: a function in __TEXT,__hot and a global in __DATA,__tbl → otool -l shows the sections; the program runs → 42.
031-opcode.mc: #opcode for movz used in a function that returns 42 without return.
032-svc.mc: #include "../lib/sys_svc.mc"; write(1, "hi\n", 3) via svc → stdout hi\n.
033-reloc.mc: reloc(BRANCH26, "_helper"); emit(0x94000000); calling a function from the same file → 42; otool -r shows the relocation.

make test green; otool -r/otool -l of the 030 and 033 .o files attached to the report.