one pure hash — capacity for invariants only, no mode variance

hemera is the cyber stack's identity function: hash(data) → particle. this proposal removes the two pieces of inherited variance — content-type in capacity, and BLAKE3-style hash modes — leaving one pure hash. supersedes capacity-typing (kept for rationale, not implemented).

context: this is the hemera half of a stack-wide type/terms cleanup. the type theory lives in soft3/specs/types.md, the vocabulary in soft3/specs/terms.md, the nox half in nox/roadmap/data-model.md, and the cross-repo execution sequence in soft3/roadmap/migration.md.

the rule

a hash mechanism earns a place in hemera only if it is a substrate invariant — a truth of the system that can never legitimately change. type and convenience modes are neither.

1. type does not go in capacity — reject capacity-typing

capacity-typing.md proposes hash_typed(bytes, TYPE_PARTICLE) — content type in state[14], a frozen genesis registry, type intrinsic to identity. its goal (distinct identity for distinct referents) is right; its mechanism is the one types.md rejects:

• a refinement (word, bool) is a proposition about a value, identity-neutral — it never touches the hash.
• a nominal type (timestamp, particle vs cyberlink) is a distinct referent, expressed as structure: pair(domain, value). different content → different particle, for free, through ordinary content-addressing. the domain marker is itself a particle, so the type is a graph node you can link to — open, not a frozen registry.

so the type-confusion capacity-typing.md worries about is solved one layer up, by structure, not by a capacity lane. state[14] stays unused. mark capacity-typing.md superseded.

2. the real capacity layout (spec vs code)

the nox spec claimed capacity[14] = atom type tag. the code never implements it. the actual 16-element state is rate[0..8] | capacity[8..16]:

state[8]   counter     chunk position in a file tree        hemera tree
state[9]   flags        FLAG_CHUNK / FLAG_PARENT / FLAG_ROOT   hemera tree   ← KEEP
state[10]  msg_length   absorbed length, bound at finalize    hemera sponge
state[11]  mode         hash / keyed / derive_key             hemera sponge ← REMOVE (§3)
state[12]  ns_min       NMT namespace lower                   hemera NMT
state[13]  ns_max       NMT namespace upper                   hemera NMT
state[14]  —            unused (capacity-typing proposed type) ← stays unused
state[15]  —            unused

state[9] (leaf/node/root) is the one genuine invariant — second-preimage safety, true forever by the mathematics of authenticated trees. it earns capacity. nox must stop writing the type tag into data[8] and the counter argument (the nox half handles this).

3. remove the sponge modes — one pure hash

state[11] carries BLAKE3's three modes — hash, keyed_hash, derive_key (sponge.rs:13-16, ported with commit 0278115 "for BLAKE3"). they are a general-purpose-library convenience, not a stack need: the cyber stack already separates concerns — hemera does identity, mudra does keyed crypto (KEM, AEAD, threshold, KDF).

evidence (survey): across the whole stack the modes have exactly one real caller — radio/iroh-relay/src/protos/handshake.rs:215,594-597 (hemera::derive_key, keyed_hash), doing challenge-keying — squarely mudra's job. (bita's derive_key is its own PBKDF2; everything else is hemera's CLI/tests/wgsl mirror.)

plan:

• move radio's handshake KDF / keyed-MAC to mudra (gate: confirm mudra exposes a KDF and a keyed-MAC — almost certainly yes; AEAD contains a MAC, a KEM stack a KDF).
• delete keyed_hash / derive_key (lib.rs:94,105) and the DOMAIN_KEYED / DOMAIN_DERIVE_* constants; drop the CLI subcommands.
• state[11] frees. per the discipline, it stays empty — capacity is governance-scarce; an empty lane is correct until a real invariant needs it.

result: hemera is hash(data) → particle and nothing else. one mode, no state[11] variance, no state[14] typing.

open question

does anything beyond radio need a fast symmetric MAC / KDF that mudra would not cover? if so, it lives in mudra, not hemera. confirm before deleting the modes.