Accept relations the DD session did not start with

[ajwdev]

Summary

Make the DD session accept facts for relations it did not start with,
and treat declared-but-empty relations as inputs, instead of silently
dropping facts or failing with "not found in rels".

Problem

The DD worker only creates input handles for relations known at spawn.
Command::Insert/Remove do if let Some(h) = handles.get_mut(&rel),
so facts for any other relation vanish. Separately, a relation that is
declared but never derived or populated is not an input at all.

Reproduced on upstream/main with --backend dd against testdata/:

$ printf '%s\n' '+ brand_new("x").' 'brand_new(X)' '::quit' | pallograph ... --backend dd
Asserted.
No entries for 'brand_new'.

$ printf '%s\n' '?- nodepool_taint(A,B,C,D)' '::quit' | pallograph ... --backend dd
Error: dd backend cannot evaluate this rule:
rule for `_0`: relation `nodepool_taint` not found in rels

The interpreter backend returns brand_new("x") for the first and
No results. for the second.

Fix

• DdSession records the relations it has an input handle for
  (inputs) and exposes has_input(&self, rel) -> bool.
• Engine::add_fact: when a session is live and the relation has no
  input handle, push the fact to the EDB and rebuild the session
  (via a small private rebuild_session, which reports errors on
  stderr) rather than dropping it.
• build_strata: relations that are Decl-ed, not derived by any rule,
  and not already extensional are also session inputs.

After the fix, both cases match the interpreter:

Asserted.
  brand_new("x")
Found 1 entries:

No results.

Known limitation

Adding a rule over a relation that was created after the session
started (for example ?- brand_new(X) right after + brand_new("x").)
still fails on DD with "not found in rels". That goes through
add_rule / add_idb, not add_fact, and is left for a follow-up.

Testing

• engine::tests::dd_add_fact_new_relation_is_visible: add_fact on a
  brand-new relation is visible via query_live and evaluate, and
  matches the interpreter. Fails without the has_input/rebuild change.
• engine::tests::dd_declared_but_empty_relation_is_queryable: a rule
  over a declared-but-empty relation compiles on DD and yields no rows.
  Fails without the build_strata change (relation lonely not found in rels).
• cargo fmt --check, cargo clippy --all-targets --locked -- -D warnings
  and cargo test --all-targets --locked all pass on stable.

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