Add validation constraints to FitConfig Card 2 integer fields

[KedoKudo]

Summary

FitConfig defines six optional integer fields that map to SAMMY Card Set 2 parameters, but none have validation constraints:

iptdop: Optional[int] = Field(default=None, description="Grid enhancement for Doppler broadening")
iptwid: Optional[int] = Field(default=None, description="Grid enhancement for resonance tails")
ixxchn: Optional[int] = Field(default=None, description="Special channel skip or ENDF ZA")
ndigit: Optional[int] = Field(default=None, description="Digits for compact covariance output")
idropp: Optional[int] = Field(default=None, description="Percent threshold for zeroing covariances")
matnum: Optional[int] = Field(default=None, description="ENDF material number")

Currently any integer value is accepted. Invalid values are only caught when SAMMY runs, which gives poor error messages and wastes compute time.

Suggested Approach

• Cross-reference the SAMMY manual for each field's valid range and semantics
• Add appropriate Pydantic constraints (e.g., ge=0, le=100, Literal for enumerated values)
• Add unit tests verifying that out-of-range values are rejected at config construction time

Location

src/pleiades/sammy/fitting/config.py, lines 31-40

Context

Identified during review of PR #220. Not a blocker — None defaults mean SAMMY's own defaults are used, and SAMMY will reject truly invalid values at runtime. But early validation would improve the user experience.

Related

• PR Fix/213 integration of fitconfig #220 (Fix/213 integration of fitconfig)
• SAMMY manual Card Set 2 specification