Reject decimal points after hex float literals

[Keno]

Hex float literals like 0x1p3 should not be allowed to be followed by decimal digits or decimal points, as this creates confusing implicit multiplication that looks like a malformed literal. This change makes the parser reject expressions like 0x1p3.2 (previously parsed as 0x1p3 * 0.2) as invalid numeric constants, consistent with the existing restriction on juxtaposing hex integer literals.

Fixes #60189. Written by Claude (obviously - since it's set as the author - trying out the new web thing).

I figure this situation is rare enough and bad enough that we should just change this unconditionally across syntax versions, but if preferred, we could make use of syntax evolution for this (although we'd have to start versioning the lexer).

[Keno]

I figure this situation is rare enough and bad enough that we should just change this unconditionally across syntax versions, but if preferred, we could make use of syntax evolution for this (although we'd have to start versioning the lexer).

Claude tells me flisp already has this fix, so indeed this is a bugfix, not a minor change.

keno@demeter6:~$ JULIA_USE_FLISP_PARSER=1 julia
        Info A new `nightly` version is available. Install with `juliaup update`.
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.14.0-DEV.81 (2025-11-13)
 _/ |\__'_|_|_|\__'_|  |  Commit f5f4c0174ed (8 days old master)
|__/                   |

julia> 0x1p3.2
ERROR: syntax: invalid numeric constant "0x1p3."
Stacktrace:
 [1] top-level scope
   @ REPL:1

julia> 
keno@demeter6:~$ julia
        Info A new `nightly` version is available. Install with `juliaup update`.
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.14.0-DEV.81 (2025-11-13)
 _/ |\__'_|_|_|\__'_|  |  Commit f5f4c0174ed (8 days old master)
|__/                   |

julia> 0x1p3.2
1.6