nx_dec_emit.nx

nx_dec_emit.nx -- CANONICAL non-negative i64 -> decimal ASCII. Per the bit-level no-tool-proliferation cardinal, this is the single source for emitting a non-negative integer as decimal digits. Four pre-existing inline implementations (nxembed::emit_dec, nx_q14_format::nx_int_to_decimal, nx_tptp_emit::nx_int_to_decimal, ipaddr::ip_emit_dec) predate this primitive and are queued for migration to compose it (Task: dec-emit-consolidate). New code MUST compose this; never inline another copy. Pure formatting -- no syscalls, no allocations, no imports. Writes digits directly into caller-owned `out` at offset `off`. Returns the number of bytes written (always positive). Caller responsibilities: - out must have at least 20 bytes free at off (max i64 in decimal is 19 digits + room for the loop's reverse step). - n must be non-negative (callers needing signed-with-minus should write '-' themselves then pass abs(n)). Algorithm: write digits LSB-first into out[off..off+k] then reverse in place. No scratch buffer required.