Clarify reserved cde-n profile wording

[tyche-dev]

Summary

• replaces the reserved cde-n description CDE/dCBOR normalization; SHA-256 with the already accepted neutral wording Deterministic CBOR canonicalization profile; SHA-256 in the algorithm table, IANA table, and live REGISTRY.md
• clarifies that a reserved registry entry binds only its token; its summary remains provisional until definition
• expands §3.2 to defer the deterministic encoding profile, float width, and exceptional-value policy, and states that cde-n is not currently defined as CDE, dCBOR, or the CBOR WG deterministic-serialization work
• regenerates the committed XML and text artifacts

Closes #17.

Why

The current description names two different profiles and pre-attributes semantics that have not been selected. The replacement removes that false attribution without silently choosing shortest-width versus binary64 floats before written encoder evidence arrives from libp2p.

The entry remains Reserved. This PR does not define cde-n, choose a float policy, or claim interoperability.

Validation

• make -B -C spec ... all using kramdown-rfc2629 1.7.39 and xml2rfc 3.34.0
• repeat build produced byte-identical XML and TXT (SHA-256 stable)
• python3 -m pytest lib: 32 passed
• python3 .github/check_vectors.py vectors/: 0 FAILED
• python3 .github/neutrality_scan.py --self-test: OK
• git diff --check: clean

[tyche-dev]

@StevenMih PR #22 is ready and its current required checks are green. The repository ruleset blocks merge until one approving review from a reviewer with write access. Please review and approve exact head 2bc7b55810647cc593aa4b4966362c7533477209; it can be merged immediately afterward.