feat(M6a): semantic-cluster fixes ($$ root, $keys/$spread, undefined arithmetic, object keys)

[flearc]

Summary

M6a fixes four independent semantic divergences that all teach the same jsonata invariant — an empty sequence IS undefined, and undefined propagates by omission, not by empty-string/empty-array. It's the first of the M6 "structural polish" split (M6b = parser [] keepArray; #$var later).

• C-1 $$ root context — bind the input under the name "$" at the top frame, so $$ resolves through the normal variable chain from any nested scope (it was never bound → all $$ forms returned undefined). $ (current context) is unaffected.
• B-3 $keys/$spread — $keys of an empty result is now undefined (not []); $spread echoes a scalar argument and an empty $spread is undefined.
• C-4a undefined arithmetic — x + undefined → undefined (and unary -undefined → undefined), matching jsonata's exact operand ordering: both defined-non-number type errors (T2001/T2002) fire before the undefined short-circuit; unary - on a non-number raises D1002.
• C-3 object-literal key rules — an undefined key skips the pair, a non-string key raises T1003, an undefined value omits the pair, and a duplicate key raises D1009 (detected at key-time, independent of value).

Results

• Official jsonata suite: 1153 → 1174 / 1682 (+21, 69.0% → 69.8%), zero regressions. Gains across conditionals, function-keys, numeric-operators, object-constructor, function-spread, and 5 more groups.
• 448/448 unit tests green.

Verification

An adversarial fidelity review against a jsonata-js v2.2.1 oracle (~100 probes) found three suite-invisible in-scope divergences — $spread({}), duplicate-key-with-undefined-value, and unary-minus error code — all fixed in 204ab63. (The official suite runner accepts any structured error without comparing codes, so error-code fidelity is checked only by the oracle.)

Deferred (out of scope): singleton-unwrap of builtin collection results ($keys({a:1}) → ["a"] vs jsonata "a") — an architectural sequence-semantics concern for a dedicated later milestone.

Test Plan

• busted full unit suite — 448/448
• scripts/run-suite.sh — 1174/1682, zero regressions
• Adversarial fidelity review vs jsonata-js v2.2.1 oracle

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