[HS3] Add structured interpolation support for HistFactory modifiers

[Phmonski]

Summary

This PR adds bidirectional RooFit support for the structured interpolation representation introduced in hep-statistics-serialization-standard/hep-statistics-serialization-standard#103.

RooFit now exports interpolation behavior using the descriptive HS3 {type, in, out} structure instead of RooFit-specific integer codes. Import remains backward compatible with legacy integer interpolation fields and files relying on the previous implicit defaults.

Main changes

• Add an internal structured interpolation descriptor with validated type, in, and nullable out fields.
• Add class-specific mappings for FlexibleInterpVar and PiecewiseInterpolation.
• Export a channel-level default_interpolation, selected from the most common interpolation behavior across normsys and histosys modifiers.
• Omit modifier-level interpolation when it matches the channel default.
• Emit an explicit structured interpolation object for modifiers that differ from the default.
• Prefer {type: mult, in: poly6, out: exp} when frequencies are tied, followed by deterministic descriptor ordering.
• Omit default_interpolation when a channel has no eligible modifiers.
• Compare duplicate modifiers by interpolation behavior rather than their raw RooFit codes, allowing semantic aliases such as FlexibleInterpVar codes 4 and 5.
• Add a PiecewiseInterpolation constructor accepting one interpolation code per parameter. This allows mixed histosys interpolation codes and repeated parameters to round-trip faithfully.

No interpolation mathematics or persistent class data layouts are changed.

Supported mappings

HS3 interpolation	PiecewiseInterpolation	FlexibleInterpVar
{add, poly1, null}	0	0
{mult, exp, null}	1	1
{add, poly2, poly1}	2; legacy 3 alias	2; legacy 3 alias
{add, poly6, poly1}	4	unsupported
{mult, poly6, exp}	5	4 or 5; imported canonically as 4
{mult, poly6, poly1}	6	unsupported

The class-dependent interpretation is important because RooFit code 4 has different semantics in PiecewiseInterpolation and FlexibleInterpVar.

Import compatibility

Interpolation is resolved in the following order:

1. Modifier-level structured object or legacy integer.
2. Channel-level default_interpolation.
3. The legacy class-specific implicit code 4.

Structured objects must contain all three fields, including an explicit "out": null where appropriate. Malformed, unknown, and class-unsupported interpolation descriptions produce contextual errors identifying the channel, sample, and modifier.

RooFit exports only the new structured representation.

Tests

The tests cover:

• Every supported mapping and legacy alias.
• Canonicalization of legacy aliases.
• Modal channel defaults and deterministic ties.
• Explicit modifier overrides.
• Channels without eligible modifiers.
• Mixed histosys codes in one PiecewiseInterpolation.
• Numerical round trips at interior, anchor, and extrapolation points.
• Legacy integer fields and implicit defaults for both modifier classes.
• Malformed, unknown, and class-unsupported interpolation objects.
• Duplicate-modifier canonicalization using semantically equivalent codes.
• Repeated parameters with per-entry PiecewiseInterpolation codes.
  @cburgard @stalbrec

[github-actions]

Test Results

    23 files      23 suites   3d 17h 51m 52s ⏱️
 3 874 tests  3 874 ✅ 0 💤 0 ❌
79 716 runs  79 716 ✅ 0 💤 0 ❌

Results for commit ad9e6ac.