native: the seed goes down as the word the firmware reads

[A13xB0]

Not the radio. On the native build radio_new_identity() is the host variant's own HostRNG(g_identity_seed), and the identity is a pure function of --seed; RadioLib's randomByte() never enters it (that path is the board variants', which #556 handled). Run by hand with four seeds, the same binary gives four keys, as the issue found.

What destroys the seed is the parser. bridge/main.cpp reads --seed with strtoul into a uint32_t. Where a long is 64 bits (Linux, macOS) the cast keeps the low word and every node differs. Where it is 32 bits, every Windows target, strtoul saturates at ULONG_MAX on overflow. The engine strides seeds by 0x9E3779B97F4A7C15, so every node past the first carries a seed above 2^32, and all of them arrived as 0xFFFFFFFF. Node 0's seed is the bare run seed, which fits: the one node apart. It reproduces across releases because both shipped tags carry the line, and across fixtures because it does not depend on the scenario.

The fix is on our side of the command line: the seed goes down as its low 32 bits, which is all the firmware ever read. No identity changes on Linux or macOS, and the published binaries are right without a rebuild. A test pins that every seed the engine can produce goes down as a number a 32-bit parser cannot saturate on, and that two nodes of one run still get two. MeshBench/meshcore-native#21 widens the parser to 64 bits for anyone running the binary by hand.

Not verified on Windows from here: this is a code-reading conclusion with the exact arithmetic signature the issue measured (index 0 survives, every other node collapses to one value). The Linux nodefs on this machine holds 375 distinct identities for a 375-node run, which is what the low-word behaviour predicts.

Found on the way, filed as #720 (every native node draws the same radio noise, so every fast_rng is seeded identically) and #721 (identities keyed on node index, so node 0 of every fixture is the same node).

Closes #712

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