nxtask_store.nx

nxtask_store.nx -- persistence layer for nxtask (beads-equivalent). Provides buffer round-trip of a TaskGraph + convenience wrappers that go through fs.nx for disk persistence. Single-file binary format; zero runtime deps beyond syscalls + fs. Format v1 -- magic "NXT0": 4 magic ("NXT0", little-endian bytes) 8 n_tasks (i64 LE) repeat n_tasks: 8 state 8 created_ns 8 updated_ns 8 desc_len desc_len desc bytes (no trailing NUL) 8 claimed_by_len claimed_by_len claimed_by bytes (omitted when 0) 8 summary_len summary_len summary bytes (omitted when 0) 8 n_blocked 8*n_blocked blocked_by task indices (i64 LE each) Design notes: * We store strings inline rather than via a separate string pool because each task's text is small (<2 KB) and the whole file typically fits in one page. A pool only pays off when many tasks share identical text, which isn't the nxtask workload. * id_hash is NOT persisted. v0.0.1 uses counter-hash which is a function of the task's position + creation order, both of which are recovered at load time. v0.1.0 will persist a real sha256(desc + created_ns) once we switch to content-addressed ids. * Little-endian is the RISC-V default so this is the native byte order. No byte-swap needed. This commit ships the buffer API only. An fs_save/fs_load wrapper