fix: RYC metadata and pandas warnings (v5.5 sync residuals)

[brightyorcerf]

Linked issue

• Closes [Task] Fix v5.5 sync residuals: RYC metadata gap and pandas deprecation warnings #432
• Related Absorb MUIO v5.5 changes #422

Existing related work reviewed

• Issues/PRs reviewed: Absorb MUIO v5.5 changes #422

Overlap assessment

• Classification: Follow-up / Stability Fix
• Overlapping items: MUIO v5.5 Baseline Sync
• Why this is not duplicate/superseded: This PR addresses specific "residuals" identified during the v5.5 sync that were intentionally left for a targeted patch to maintain a clean upstream baseline.

Why this PR should proceed

It completes the stabilization of the v5.5 migration by fixing UI-breaking metadata gaps and ensuring backend compatibility with modern pandas environments (silencing deprecation noise).

Summary

What changed:

• Metadata Correction: Updated Variables.json for the RYC group (Energy Balance shadow prices), correcting the unitRule token from EmiUnit to CommUnit.
• Constant Alignment: Appended RYC to the VAR_COMM_GROUPS array in Const.Class.js to ensure the Results engine initializes the correct view class.
• Backend Hygiene: Added explicit dtype='object' to empty DataFrame initialization in DataFileClass.py to silence FutureWarning logs during CBC post-processing.

Why:

An audit revealed that RYC display was failing for two reasons:

• Group Omission: Because RYC was missing from VAR_COMM_GROUPS, the Javascript grouping engine skipped allocating a view class entirely.
• Token Mismatch: The unitRule for EB_d incorrectly referenced EmiUnit (Emissions). Since shadow prices for Energy Balance are commodity-based, jsonLogic produced undefined when trying to apply the rule.

Result: This fix ensures shadow prices properly reflect the Currency / Commodity marginal cost ratio in the results view.
Stability: Silences persistent type-inference warnings introduced in pandas 2.1+ that cluttered the backend logs.

Validation

• Tests added/updated (or not applicable)
• Validation steps documented
• Evidence attached (logs/screenshots/output as relevant)

Documentation

• Docs updated in this PR (or not applicable)
• Any setup/workflow changes reflected in repo docs

Scope check

• No unrelated refactors
• Implemented from a feature branch
• Change is deliverable without upstream OSeMOSYS/MUIO dependency
• Base repo/branch is EAPD-DRB/MUIOGO:main (not upstream)

Exception rationale

blank

[brightyorcerf]

These are minimal-diff, high-leverage changes intended to bridge the metadata gap without introducing upstream divergence.