refactor!: remove the derive feature and fallbacks

[sephynox]

Summary

Phase 2 made tightbeam-derive a required dependency, so the derive feature enabled nothing yet still gated code, and derived error and wire types kept hand-written fallbacks that only narrow builds compiled. Removing the feature leaves one implementation per type, and Beamable now checks tightbeam's own features instead of a copy in the derive crate.

Related Issues

None

Changes Made

• The derive feature no longer exists
• Error messages come from one #[error] declaration in every build
• Beamable feature checks follow tightbeam's features directly
• The server! service error names the pooled_mux alias

Testing

• make test-all: passes
• A crate deriving #[beam(confidential)] without aead fails with the feature message

Breaking Changes

• Remove "derive" from tightbeam-rs feature lists. The derives are always available.
• tightbeam-derive drops its aead, digest, signature, and compress features. Remove any direct enable.

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Note

Medium Risk
Breaking change for downstream features = ["derive"] and tightbeam-derive feature flags; behavior is mostly compile-time/API surface with no auth or wire-format changes.

Overview
Removes the derive Cargo feature now that tightbeam-derive is always linked: Beamable, Flaggable, and Errorizable are unconditional, and the large #[cfg(not(feature = "derive"))] manual Message, error Display, and URN-spec fallbacks are deleted.

Beamable feature validation no longer uses cfg!(feature = …) inside tightbeam-derive (or mirrored derive-crate features). It emits __tb_if_aead / __tb_if_signature / __tb_if_digest / __tb_if_compress with an else { compile_error!(…) } branch so missing aead, signature, digest, or compress on tightbeam-rs fails at compile time with a clear message.

Feature wiring cleanup: builder, colony, instrument, and compress drop derive / tightbeam-derive/* deps; consumer crates and test_features.sh stop enabling "derive". The server! pooled-mux compile_error text now points at the pooled_mux build alias.

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