docs(compound): guide users to negation workaround for side compounds

[justinjoy]

Summary

Negating a side-tier compound body pattern (e.g. !event(ID, metadata(...))) is rejected at IR conversion because negated side-join lowering is not yet implemented. This is a narrow limitation — it fires only when all three hold: (1) the atom is negated, (2) the column is a side-tier compound, and (3) the body pattern destructures it with functor syntax. Negating presence (!event(ID, _)), inline compounds, and positive destructuring all work today.

Crucially, expressive power is unaffected: extracting the compound fields in a positive rule and negating the resulting flat relation is fully supported and stratifies naturally. This PR makes that workaround discoverable instead of leaving users stuck on an opaque error.

Changes

• wirelog/ir/program.c — extend the IR-conversion error message to name the workaround and point at the docs.
• docs/COMPOUND_TERMS.md — add a "Negation and side compounds" section documenting the exact trigger conditions, the unsupported form, and the supported positive-extraction pattern; link it from the Errors table and TOC.

Behavior / risk

Behavior is unchanged — the pattern is still rejected. Only the log string and documentation change.

Testing

• Clean build compiles.
• meson test wirelog_easy wirelog_advanced pass (includes the negated-side-compound rejection test and side-compound binding tests).
• Full suite: 249 pass / 10 skip; the single failure is sbom_snapshot (dependency-manifest baseline check), which is environmental and unrelated to a log-string + docs change.

Follow-up

Full negated side-join lowering implementation remains deferred; promote when real demand accumulates or language orthogonality becomes a release goal.