[petlenz]

Closes #365.
Closes #366.

Two independent grammar/action defects, one commit each.

#365 — unary minus rejected tensor and t2s operands

replace_top (src/numsim_cas/parser/actions.h) called require_scalar, so -A, -trace(A) and -det(A) raised type_mismatch_error: operator expects scalar operands even though negation is defined in all three domains (expression_holder::operator-, with neg_fn tag_invokes for tensor and t2s). Users had to write -1 * A.

It now visits the operand variant and negates whichever expression alternative it holds; only index_list_value is rejected, with a message matching the existing bracket-list wording used by the binary operators.

#366 — negative exponents unparseable

power_tail was seq<ws, caret_op, ws, power>, and power cannot start with -, so 2^-3 and x^-1 failed. The parser accepted the left operand and then choked, producing the misleading parse error at 1:2: parse error matching tao::pegtl::eof.

The right operand is now unary, which recurses back into power.

Precedence decisions (unchanged, and pinned by tests):

Input	Parse	Why
2^3^2	2^(3^2) = 512	unary → power keeps the right recursion
2^-3^2	2^(-(3^2)) = 2⁻⁹	the ^ tail binds before negating the exponent
-x^2	-(x^2)	decided at the unary level above power; left operands stay at primary
2^3*4	(2^3)*4 = 32	^ still binds tighter than *
2^-3*4	(2^-3)*4 = 0.5	same

x^-3 and x^(-3) now produce structurally identical nodes (hash and deep equality both checked).

Tests

10 new in ParserGrammar: NegativeExponentLiteral, NegativeExponentOnIdentifier (incl. x^-2.5), NegativeExponentMatchesParenthesizedForm, NegativeExponentKeepsRightAssociativity, UnaryMinusOnTensorMatchesCppNegation, UnaryMinusOnT2sMatchesCppNegation, UnaryMinusOnTensorSum, DoubleUnaryMinusOnTensorCancels, UnaryMinusOnDetIsT2s, UnaryMinusRejectsBracketList. The tensor/t2s cases compare the parsed expression against the C++-built node with deep ==, not just "parsing succeeded".

Negative control: against unfixed sources, 9 of the 10 fail (the bracket-list rejection passes both ways by design). The pre-existing precedence pins — PowerRightAssociative, PowerBindsTighterThanMul, PowerBindsTighterThanUnaryMinus, UnaryMinusBindsTighterThanMul, DoubleUnaryMinusCancels, BinaryMinusVsUnaryMinusAfterBinary — pass throughout; none was modified.

Error quality: verified against main that genuinely invalid inputs are unchanged — 2^, 2^*3 and x^- produce the same message and position as before (the internal PEGTL rule name leaking there is pre-existing, #367's scope).

Full parser-enabled suite: 2601/2601 (gcc-14 Debug, NUMSIM_CAS_BUILD_PARSER=ON). clang-format-18 clean.