fix(core): reject partial and empty numeric strings

[Railly]

The numeric validator used parseFloat, so "123abc" and "99bottles" passed as valid numbers. Switching to Number() alone would let "" and whitespace-only strings pass, since both coerce to 0. The validator now requires a non-empty string that Number() parses fully, with tests for both cases.

Recreates the numeric half of #306 by @fix2015, credited as co-author. The $truncate half changes documented behavior (existing tests expect the suffix after length), so it is left out of this PR.

[vercel]

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